Tropical Storm Sam

Post Tropical Sam Track 0900 Hours October 4 2021
Post Tropical Sam Track 0900 Hours October 4 2021

Tropical Storm Sam Wind Speed FieldPost-Tropical SamNWS National Hurricane Center Miami FL 900 AM GMT Tue Oct 05, 2021 (see Tuesday video below)

Sam’s low-level circulation center has become exposed on the south side of an east-to-west oriented cloud band containing cloud tops that have warmed to around -50 deg C. The cyclone’s overall cloud pattern in satellite imagery resembles that of an occluded extratropical low pressure system, and surface analyses from the NOAA Ocean Prediction Center indicate that Sam’s inner-core is now interacting with a frontal system. Furthermore, the system is located over sea-surface temperatures of around 14 deg C. Although the system likely still has a warm-core thermal structure in the mid- and upper-levels of the troposphere based on earlier 04/2321Z AMSU-A/-B data, the low-level circulation now is comprised of an extensive field of cold-air stratocumulus clouds. Based on these data, Sam has made the transition to an extratropical cyclone over the far north Atlantic.

The initial wind speed has been lowered to 70 kt based on a standard decay rate.  The initial motion estimate is northward or 010/20 kt.

Extratropical-Sam has been moving north-northeastward at 35 kt since the previous advisory, but the latest global and regional models are all in very good agreement that the system will slow down significantly during the next 6-12 hours as it interacts with an approaching baroclinic mid- to upper-level trough/low. By late Tuesday, the post-tropical cyclone should turn east-northeastward, and then make a counter-clockwise loop southwest of Iceland on Wednesday. By Thursday, the large cyclone is expected to move east-northeastward or eastward and pass very near the southern coast of Iceland on days 4 and 5 as a weakening extratropical low pressure system. The new NHC track forecast is very similar to the previous advisory track, and lies close to the tightly packed simple- and corrected-consensus track models. Extratropical-Sam is expected to maintain hurricane-force winds for the next 12-18 hours, followed by a slow weakening trend as the cyclone loses its baroclinic forcing. However, Post-Tropical-Sam is still expected to pass close to Iceland in the 96-120-h period as an expansive low pressure system producing gale-force winds over a very large area.

This is the last advisory issued by the National Hurricane Center on Sam.

FORECAST POSITIONS AND MAX WINDS

INIT  05/0900Z 51.0N  39.4W   70 KT  80 MPH - Post Tropical (ENE St. John's,NF Canada)
 12H  05/1800Z 52.1N  38.9W   65 KT  75 MPH - Post Tropical (ENE St. John's,NF Canada)
 24H  06/0600Z 52.8N  35.1W   60 KT  70 MPH - Post Tropical (ENE St. John's,NF Canada)
 36H  06/1800Z 54.3N  30.0W   55 KT  65 MPH - Post Tropical (ESE Nanortalik,Greenland)
 48H  07/0600Z 57.9N  24.6W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  07/1800Z 61.8N  23.9W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
 72H  08/0600Z 62.7N  27.4W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
 96H  09/0600Z 61.7N  26.2W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)
120H  10/0600Z 63.6N  15.0W   40 KT  45 MPH - Post Tropical (ESE Hof,Iceland)

NWS National Hurricane Center Miami FL 900 PM GMT Mon Oct 04, 2021 

Sam’s cloud pattern has finally begun to degrade as it moves over the cooler waters of the north Atlantic. The cloud tops have warmed and the eye is no longer discernible in infrared imagery, but banding features still remain well defined in all quadrants. Dvorak T- and CI-numbers from both SAB and TAFB have continued to decline and a blend of the latest subjective and objective estimates yields an initial intensity estimate of 80 kt for this advisory. Scatterometer data that arrived just after the release of the previous advisory showed that Sam’s wind field has continued to expand and the latest radii estimates are based on those data.

Sam should gradually weaken this evening as it moves over progressively colder waters and into an area of strong southwesterly upper-level winds. However, an approaching mid-latitude trough should result in the system’s quick transition into a powerful extratropical cyclone overnight. The post-tropical cyclone is forecast to maintain hurricane-force winds for until late Tuesday, but gradual weakening should occur after that time as the system occludes and baroclinic forcing decreases. It is not entirely clear as to whether Post-Tropical Sam will remain the dominant low (GFS solution) or if it will be absorbed by another extratropical low (ECMWF solution) later in the week. As a compromise, the new official forecast calls for absorbed by day 5 which is between the solutions from those typically reliable global models.

Sam is moving briskly northeastward or 035/31 kt. The cyclone is currently caught in strong southwesterly flow ahead of the aforementioned mid-latitude trough and a rapid northeastward motion is expected overnight. After that time, the post-tropical cyclone is forecast to slow down as it merges with the cut-off low. By late Tuesday, the post-tropical cyclone should turn east- northeastward before it rotates cyclonically around the eastern portion of another trough/cut off low over the north Atlantic. The latest NHC forecast is a blend of the GFS and ECMWF models and their respective ensemble means.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland through early Tuesday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  04/2100Z 45.1N  42.5W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
 12H  05/0600Z 49.0N  39.8W   70 KT  80 MPH - Post Tropical (ENE St. John's,NF Canada)
 24H  05/1800Z 50.9N  39.9W   65 KT  75 MPH - Post Tropical (ENE St. John's,NF Canada)
 36H  06/0600Z 50.5N  36.0W   55 KT  65 MPH - Post Tropical (ENE St. John's,NF Canada)
 48H  06/1800Z 52.5N  30.0W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  07/0600Z 56.5N  25.0W   45 KT  50 MPH - Post Tropical (ESE Nanortalik,Greenland)
 72H  07/1800Z 60.0N  25.5W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
 96H  08/1800Z 60.0N  32.0W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
120H  09/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Mon Oct 04, 2021 

Sam remains a formidable high-latitude hurricane this morning. An earlier 1102 UTC GMI microwave overpass continued to depict concentric eyewalls with a fairly well-defined inner-eye that was open over the southwestern quadrant. Although there has been a generally warming of the cloud tops in infrared satellite imagery, the eye remains apparent and the system is still fairly symmetric. A blend of the subjective Dvorak T- and CI-numbers from both SAB and TAFB yield an intensity of 85 kt, which is used as the initial intensity for this advisory.

Now that Sam has moved north of the north wall of the Gulf Stream it will be moving over progressively colder waters and into much higher vertical wind shear conditions by this evening, which should cause some gradual weakening. However, the interaction of the cyclone with a strong upper-level trough is likely to lead to the rapid transition of Sam into a powerful extratropical cyclone later this evening. The post-tropical cyclone is expected to maintain hurricane-force winds until late Tuesday. Gradual weakening should occur after that time as the baroclinic forcing decreases, and the system is forecast to spin down slowly over the north Atlantic later this week.

Sam continues to accelerate northeastward and is now moving at 045/28 kt. A faster northeastward motion is forecast through tonight as Sam moves within the strong southwesterly flow ahead of a mid-latitude trough. The post-tropical cyclone is forecast to slow down on Tuesday as it interacts with the trough/cut-off low. After that time, the system is forecast to turn east-northeastward, and then rotate cyclonically around the eastern flank of yet another cut-off low later in the week. The dynamical model guidance remains in generally good agreement on the overall track forecast scenario, albeit for some forward speed variations. The latest official forecast is similar to the previous advisory and lies near the TVCA multi-model consensus in deference to the aforementioned along-track differences.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland for the next day or so. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  04/1500Z 42.6N  45.7W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 12H  05/0000Z 46.5N  41.9W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)
 24H  05/1200Z 50.0N  40.5W   70 KT  80 MPH - Post Tropical (ENE St. John's,NF Canada)
 36H  06/0000Z 50.3N  38.3W   60 KT  70 MPH - Post Tropical (ESE Nanortalik,Greenland)
 48H  06/1200Z 51.4N  33.2W   55 KT  65 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  07/0000Z 54.3N  27.7W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
 72H  07/1200Z 58.3N  24.8W   50 KT  60 MPH - Post Tropical (WSW Vík,Iceland)
 96H  08/1200Z 61.0N  29.7W   50 KT  60 MPH - Post Tropical (WSW Vík,Iceland)
120H  09/1200Z 60.0N  29.8W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

NWS National Hurricane Center Miami FL 1100 PM AST Sun Oct 03, 2021

Sam still had a surprise up its sleeve tonight, with its eye becoming warmer on satellite images along with a stronger eyewall. Microwave data from earlier today indicated that Sam has been undergoing a concentric eyewall cycle, and it appears that the inner eyewall has become better defined. Regardless, it is not every day you see a hurricane with that clear of an eye near 40N, and the intensity estimates the evening range from 90-100 kt. Given the concentric eyewalls, the initial wind speed is conservatively raised to 90 kt, but it could be higher.

High-resolution NOAA OISST data indicate that Sam is moving near a warm eddy along the north wall of the Gulf Stream. This favorable factor, in addition to low shear, should keep Sam’s weakening to a minimum in the near term. Later on, although the shear increases rapidly and SSTs fall quickly, Sam is expected to transition into a powerful extratropical cyclone late Monday or early Tuesday due to a mid-latitude trough interaction well east of Newfoundland. This should cause extratropical Sam to maintain hurricane-force winds until early Wednesday. A slow weakening is expected thereafter as it slowly spins down as an occluded low. No significant changes were made to the previous forecast.

Sam is moving faster, now 050/17 kt. A northeastward motion and continued acceleration is forecast through Monday as the cyclone gets caught in southwesterly flow ahead of the mid-latitude trough. The tropical cyclone is expected to be the main surface low center as it merges with the trough in a couple days, hooking briefly to the left. Thereafter, the system should resume a northeastward motion and gradually turn northward and even westward at long range as it moves around another trough. Model guidance is close to the previous cycle, even with the loopy track, and the new NHC forecast is basically just an update of the last advisory.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  04/0300Z 39.3N  51.2W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 12H  04/1200Z 41.6N  47.3W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 24H  05/0000Z 46.5N  42.1W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)
 36H  05/1200Z 50.4N  40.3W   70 KT  80 MPH - Post Tropical (ENE St. John's,NF Canada)
 48H  06/0000Z 50.8N  38.3W   65 KT  75 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  06/1200Z 51.6N  33.3W   55 KT  65 MPH - Post Tropical (ESE Nanortalik,Greenland)
 72H  07/0000Z 54.5N  28.2W   45 KT  50 MPH - Post Tropical (ESE Nanortalik,Greenland)
 96H  08/0000Z 61.0N  28.0W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
120H  09/0000Z 61.5N  32.0W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

NWS National Hurricane Center Miami FL 500 PM AST Sun Oct 03, 2021 

Sam is a resilient hurricane despite having just crossed the 26 degrees C isotherm. An eye remains readily apparent in satellite imagery, while microwave images show a symmetrical concentric eyewall structure. Recent scatterometer data revealed that the tropical-storm-force wind field continues to expand, with these winds now extending 200 n mi or greater in all quadrants. A blend of the latest Dvorak intensity estimates from TAFB, SAB, and the UW-CIMMS ADT supports maintaining an initial advisory intensity of 85 kt. Gradual weakening is expected over the next 36 h as Sam moves over progressively cooler waters. However, the wind field is forecast to continue expanding. On Monday, the cyclone is expected to interact with a strong mid-level trough moving off the coast of Newfoundland which should cause Sam to transition into a powerful extratropical cyclone. With a lack of baroclinic forcing once Sam becomes vertically stacked with the trough in 48 h, the extratropical cyclone is expected to steadily weaken over the northern Atlantic through late this week. The latest NHC intensity forecast is little changed from the previous one, and is near the various model consensus solutions.

Sam has begun to accelerate and is now moving 060/14 kt. A northwestward motion and continued acceleration is forecast through Monday as the cyclone gets caught in southwesterly flow ahead of the mid-level trough. Just before Sam merges with the trough in a couple of days, the cyclone should slow down as it pivots around the eastern flank of the trough. Thereafter, the merged system should continue northeastward then northward in the flow around a larger mid- to upper-level trough over the far northern Atlantic. The model guidance is in very good agreement on this track scenario, and the latest NHC track forecast remains near the previous one.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  03/2100Z 38.6N  52.8W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 12H  04/0600Z 40.3N  49.7W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
 24H  04/1800Z 44.2N  44.3W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)
 36H  05/0600Z 49.0N  40.4W   70 KT  80 MPH - Post Tropical (ESE St. John's,NF Canada)
 48H  05/1800Z 50.8N  39.5W   65 KT  75 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  06/0600Z 51.5N  35.7W   55 KT  65 MPH - Post Tropical (ESE Nanortalik,Greenland)
 72H  06/1800Z 53.2N  31.2W   45 KT  50 MPH - Post Tropical (ESE Nanortalik,Greenland)
 96H  07/1800Z 59.4N  27.2W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
120H  08/1800Z 61.2N  27.5W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

NWS National Hurricane Center Miami FL 1100 AM AST Sun Oct 03, 2021 

Sam’s cloud pattern remains quite symmetric for a hurricane moving into the higher latitudes. The eye has become more apparent in both visible and infrared satellite imagery over the past few hours. A very timely 1155 UTC GMI microwave image also reveals a well-defined inner eye that was surrounded by an outer ring of convection at a much larger radius. A blend of the T- and CI-numbers from the lastest subjective Dvorak classifications yields an initial intensity of 85 kt, this is also supported by earlier UW/CIMSS SATCON estimates.

The vertical shear over Sam is expected to remain low for the next 6-12 hours and only gradual weakening is anticipated while the system moves over gradually decreasing SSTs. Later tonight, Sam is expected to move over much cooler waters north of the north wall of the Gulf Stream and into an area of increasing southwesterly shear, which should result in continued weakening. However, a strong mid-tropospheric trough approaching Sam from the west on Monday is expected to result in a fairly quick transition of Sam to a large and powerful extratropical low. The low is forecast to gradually weaken over the far north Atlantic during the middle to latter portions of the week as the baroclinic forcing decreases. The latest NHC intensity forecast is similar to the previous advisory and is a blend of the various intensity aids during the early portion of the period, but leans toward the global model guidance during the post-tropical phase.

Sam is moving northeastward or 050/14 kt. The cyclone should continue to move northeastward in the flow between a strong subtropical ridge over the eastern Atlantic and mid-latitude trough over Atlantic Canada. As this trough approaches Sam from the west, a much faster northeastward motion is expected by Monday and Monday night. Around midweek, the post-tropical cyclone is forecast to slow down as it rotates around a large cut-off low over the north Atlantic. After that time, the low is forecast to resume a northeastward motion before it slows once again to the southwest of Iceland. Given the future complex interactions of the post-tropical low with the cut-off low and trough over the north Atlantic, the track guidance is in surprisingly good agreement, and the NHC forecast is near a blend of the ECMWF and GFS global models.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 37.7N  54.6W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 12H  04/0000Z 39.3N  51.9W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
 24H  04/1200Z 42.4N  47.0W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)
 36H  05/0000Z 47.0N  41.7W   70 KT  80 MPH - Post Tropical (ESE St. John's,NF Canada)
 48H  05/1200Z 50.9N  39.8W   65 KT  75 MPH - Post Tropical (ESE Nanortalik,Greenland)
 60H  06/0000Z 50.9N  38.5W   60 KT  70 MPH - Post Tropical (ESE Nanortalik,Greenland)
 72H  06/1200Z 51.5N  33.5W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
 96H  07/1200Z 59.2N  27.5W   45 KT  50 MPH - Post Tropical (WSW Vík,Iceland)
120H  08/1200Z 60.5N  31.0W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

\NWS National Hurricane Center Miami FL 1100 PM AST Sat Oct 02, 2021

Sam continues to gradually weaken. Conventional satellite imagery indicates that the eye has disappeared and that the cloud tops near the center are gradually warming. Various objective and subjective satellite intensity estimates are in the 90-102 kt range, while scatterometer and experimental synthetic aperture radar data suggest lower winds than that. The initial intensity is reduced to a possibly generous 100 kt.

The initial motion is northeastward or 040/14 kt. A faster motion toward the northeast is expected during the next couple of days as Sam is steered by southwesterly flow between a deep-layer ridge to the east and a large mid- to upper-level low over Atlantic Canada. After Sam completes extratropical transition in about 48 h, a wobbly motion generally toward the northeast is expected as Sam interacts with one or more mid-latitude shortwave troughs. The new forecast track is similar to the previous track and lies close to the various consensus models.

The hurricane is moving toward colder water, and it is approaching a frontal zone over the north Atlantic. The cyclone should undergo extratropical transition during the next couple of days, and due to a favorable interaction with a mid-latitude trough it should weaken only slowly as it becomes a large and powerful extratropical low by 48 h. After that time, the baroclinic forcing ends and the low should gradually spin down over the far north Atlantic. The new intensity forecast has only minor adjustments from the previous forecast.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  03/0300Z 36.4N  57.1W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 12H  03/1200Z 37.7N  55.3W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 24H  04/0000Z 39.7N  51.8W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 36H  04/1200Z 42.7N  47.0W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 48H  05/0000Z 47.3N  41.9W   80 KT  90 MPH - Post Tropical (ESE St. John's,NF Canada)
 60H  05/1200Z 50.8N  39.5W   75 KT  85 MPH - Post Tropical (ESE St. John's,NF Canada)
 72H  06/0000Z 51.7N  37.9W   65 KT  75 MPH - Post Tropical (ESE Nanortalik,Greenland)
 96H  07/0000Z 54.0N  30.0W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
120H  08/0000Z 60.5N  28.5W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

NWS National Hurricane Center Miami FL 500 PM AST Sat Oct 02, 2021 

The hurricane is gradually losing strength. Satellite images show that the eye has become more cloud filled, with Sam’s ring of deep convection becoming less intense and two main towers in the southeast and northwest quadrants. Still, the tropical cyclone is quite a sight even on full-disk images with a large distinctive cloud pattern. The latest intensity estimates have decreased, so the initial wind speed is set to 110 kt, with higher uncertainty than average in this figure due to recent recon/satellite differences.

Sam wisely has avoided much shear during its 10-day journey across the Atlantic, and it could remain a major hurricane for another day or so. However, in 36 hours or so, the system should move north of the Gulf Stream, which would normally cause a dramatic weakening. But in this case, a favorable mid-latitude trough interaction should cause Sam to transition into a large and powerful extratropical low by 60 hours. After that, it loses its baroclinic forcing and should gradually spin down over the far North Atlantic southwest of Iceland. Guidance is in fairly solid agreement on this scenario, and the new intensity forecast is mostly just an update of the previous one, remaining close to the consensus.

The system continues moving northeastward at about 15 kt. Sam should move faster to the northeast by Monday as it encounters stronger wind flow between a deep-layer ridge to the east and a large mid- to upper-level low over Atlantic Canada. The track will weave leftward a bit at longer ranges due to the upper trough pulling the extratropical system north-northeastward at times, and eventually another trough has a similar tug by day 5. There are no significant changes to the previous forecast with guidance in close agreement.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  02/2100Z 35.4N  58.2W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 12H  03/0600Z 36.8N  56.5W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 24H  03/1800Z 38.6N  53.6W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 36H  04/0600Z 40.8N  49.5W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 48H  04/1800Z 44.7N  44.0W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  05/0600Z 49.3N  40.2W   75 KT  85 MPH - Post Tropical (ENE St. John's,NF Canada)
 72H  05/1800Z 51.6N  38.5W   65 KT  75 MPH - Post Tropical (ESE Nanortalik,Greenland)
 96H  06/1800Z 53.0N  32.0W   50 KT  60 MPH - Post Tropical (ESE Nanortalik,Greenland)
120H  07/1800Z 60.0N  28.0W   40 KT  45 MPH - Post Tropical (WSW Vík,Iceland)

NWS National Hurricane Center Miami FL 1100 AM AST Sat Oct 02, 2021 

Sam has been an impressive hurricane during the past several days. The latest satellite imagery shows it remains a powerful system, with a well-defined eye and symmetric cloud pattern. In fact, since the plane left overnight, the eye has actually warmed, with little change in the eyewall convection. Thus the initial intensity will stay 115 kt, above the latest satellite classifications, but consistent with the earlier aircraft data and low bias of the Dvorak technique for much of the this storm. Sam is now in the top 10 for consecutive days as a category 4 hurricane or greater in the historical record, about the same as Matthew 2016.

The hurricane is forecast to gradually lose strength during the next 36 hours or so while it remains over marginally warm waters, but in fairly light shear. In fact, some of the guidance decrease the shear overnight, which should allow Sam to keep much of its strength, provided it doesn’t undertake an eyewall cycle. The new intensity forecast is higher in the first day or so, consistent with the latest model solutions. In about 2 days, Sam will cross the north wall of the Gulf Stream, and in 3 days is expected to spectacularly transition into a large hurricane-force post-tropical cyclone over the far north Atlantic. No changes were made to the end of the intensity forecast.

Sam has turned northeastward at about 15 kt. The hurricane should accelerate later this weekend due to increasing flow between a deep-layer ridge to the east and a large mid- to upper-level low over Atlantic Canada. By midweek the system is forecast to turn more to the north-northeast as it becomes a spoke in a very large extratropical low over the far North Atlantic. The new forecast has shifted somewhat eastward during the first couple of days but ends up very near the last advisory by day 5.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, the eastern United States coast and Atlantic Canada for the next couple of days. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  02/1500Z 33.9N  59.3W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  03/0000Z 35.6N  57.8W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
 24H  03/1200Z 37.4N  55.3W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 36H  04/0000Z 39.2N  52.0W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 48H  04/1200Z 41.8N  47.3W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  05/0000Z 46.0N  42.0W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
 72H  05/1200Z 50.0N  38.5W   75 KT  85 MPH - Post Tropical (ESE St. John's,NF Canada)
 96H  06/1200Z 53.0N  33.0W   55 KT  65 MPH - Post Tropical (ENE St. John's,NF Canada)
120H  07/1200Z 59.0N  26.0W   45 KT  50 MPH - Post Tropical (WSW Sligo,Ireland)

NWS National Hurricane Center Miami FL 1100 PM AST Fri Oct 01, 2021 

The satellite appearance of Sam has degraded slightly during the past few hours, with the eye becoming more cloud filled and the eyewall cloud tops become a little less symmetric. Various subjective and objective satellite intensity estimates are in the 115-130 kt range, and based on the slight decay the initial intensity is lowered to 125 kt. An Air Force Reserve Hurricane Hunter aircraft is currently enroute to investigate Sam.

Sam is making its forecast turn toward the north-northeast with the initial motion now 020/15 kt. There is no change to the track forecast philosophy and little change to the forecast track from the previous advisory. The steering flow between a deep-layer ridge to the northeast and east of Sam and a large mid-latitude low pressure system centered over Atlantic Canada should cause Sam to turn northeastward during the next 24 h, followed by a continued northeastward motion as the system become entrained in southwesterly mid-latitude flow. The models are in fairly good agreement, although there is some spread in both the track and the speed due to model differences in the cyclone’s evolution as it becomes extratropical. The new forecast track lies close to the center of the guidance envelope and the various consensus models.

Slow weakening is expected to continue during the next 2-3 days as Sam moves over cooler sea surface temperatures and begins extratropical transition. This is likely to be complete by 72 h, with the global models suggesting that Sam will evolve into a warm-core seclusion extratropical low. Based on this and the guidance, it is likely that the cyclone will maintain hurricane strength until after the transition is done. By the end of the forecast period, the extratropical low is expected to be decaying over the far north Atlantic. The new intensity forecast lies at the upper edge of the intensity guidance.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands the Greater Antilles, the Bahamas, and Bermuda during the next couple of days. Swells are expected to reach the United States east coast and Atlantic Canada on Saturday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are possible on Bermuda during the next several hours, and a Tropical Storm Warning is in effect for the island.
FORECAST POSITIONS AND MAX WINDS

INIT  02/0300Z 31.4N  61.2W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  02/1200Z 33.5N  60.2W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  03/0000Z 35.8N  58.3W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 36H  03/1200Z 37.7N  55.6W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  04/0000Z 39.6N  52.2W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  04/1200Z 43.0N  47.3W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  05/0000Z 47.2N  43.2W   80 KT  90 MPH - Post Tropical (ESE St. John's,NF Canada)
 96H  06/0000Z 50.5N  38.0W   65 KT  75 MPH - Post Tropical (ENE St. John's,NF Canada)
120H  07/0000Z 54.5N  27.5W   50 KT  60 MPH - Post Tropical (WSW Sligo,Ireland)

NWS National Hurricane Center Miami FL 500 PM AST Fri Oct 01, 2021 

Sam remains a powerful hurricane. The eye is still very distinct and there are several mesovorticies evident in satellite imagery. In addition, microwave images show a closed and symmetric eyewall. Data from the Air Force Hurricane Hunters indicate that Sam has been maintaining its category 4 intensity. The aircraft measured peak flight-level winds at 700 mb of 152 kt, which corresponds to a 135-140 kt surface wind. However, the peak SFMR winds are around 115 kt. Dropsonde data from the aircraft show a sharp decrease in winds around the 925 mb level, which could mean that the strongest winds are not reaching the surface. The minimum pressure in the eye remains very low though at 937 mb. Based on all of this data, the initial intensity is held at 130 kt, but there is some uncertainty in that estimate. The major hurricane continues northward and the latest initial motion estimate is 355/17 kt. The large-scale pattern consists of a deep-layer ridge to the northeast of Sam and a large low pressure system centered over Atlantic Canada. The steering flow between these features should cause Sam to move northeastward at a sightly slower pace this weekend. By early next week, the deep-layer low is expected to retreat northward, which should cause Sam to turn a little more to the right. The models are in fairly good agreement, but there are some notable speed differences with the UKMET on the faster side and the GFS on the slower side of the guidance envelope. No major changes were made to the NHC track forecast, and it remains near the consensus aids. Sam is expected to slowly weaken during the next several days as it moves over progressively cooler waters and into an environment of stronger shear. The hurricane will likely begin extratropical transition in a couple of days when it crosses the 26 degree C isotherm, and this transition is expected to be completed shortly after 72 hours. All of the model guidance shows steady weakening through the forecast period, and so does the NHC forecast. This prediction is close to the IVCN, IVDR, and HCCA models in the short term and near the GFS model during the extratropical portion of the forecast. Even though Sam is likely to weaken, it is still expected to be a significant storm over the next several days.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands the Greater Antilles, the Bahamas, and Bermuda during the next couple of days. Swells are expected to reach the United States east coast and Atlantic Canada on Saturday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are possible on Bermuda beginning tonight or early Saturday, and a Tropical Storm Warning is in effect for the island.
FORECAST POSITIONS AND MAX WINDS

INIT  01/2100Z 30.0N  61.8W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  02/0600Z 32.3N  61.0W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  02/1800Z 34.8N  59.4W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 36H  03/0600Z 36.7N  57.3W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  03/1800Z 38.6N  54.2W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  04/0600Z 41.1N  50.1W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  04/1800Z 44.9N  45.6W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 96H  05/1800Z 50.5N  40.5W   70 KT  80 MPH - Post Tropical (ENE St. John's,NF Canada)
120H  06/1800Z 52.6N  30.7W   50 KT  60 MPH - Post Tropical (ENE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Oct 01, 2021

Sam remains a powerful category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale over the subtropical central Atlantic. Satellite images show that the hurricane still has a circular and quite distinct eye and closed eyewall. However, an 0827 UTC SSMIS pass indicated that a dry slot was present on the eastern side of the circulation between the eyewall and rainbands. The latest Dvorak classifications from TAFB, SAB, and CIMSS at the University of Wisconsin range from 102-115 kt. However, Air Force reconnaissance data from earlier this morning showed that the winds were much higher, and in fact, the Dvorak estimates have had a low bias compared to the Hurricane Hunter data for the past 24-36 hours.

Therefore, the initial intensity is held at 130 kt for now. Another Air Force Hurricane Hunter aircraft is scheduled to investigate Sam this afternoon. An ASCAT-A pass from around 12Z indicated that Sam’s tropical-storm-force wind radii have expanded, and the initial wind radii were adjusted based on that data. In addition, the eye of Sam passed about 60 n mi east of NOAA buoy 41049 earlier this morning. The buoy reported a minimum pressure of 998 mb, maximum winds of around 45 kt, gusts to 62 kt, and maximum seas of about 22 ft. The major hurricane has turned to the north and is now moving faster. The latest initial motion estimate is 355/18 kt. The large-scale pattern consists of a deep-layer ridge to the northeast of Sam and a large low pressure system centered over Atlantic Canada. The steering flow between these features should cause Sam to move northeastward at a sightly slower pace this weekend. By early next week, the deep-layer low is expected to retreat northward, which should cause Sam to turn a little more to the right. The models are in fairly good agreement, but there are some notable speed differences with the UKMET model being a fast outlier. The new NHC track forecast is slightly to the right of the previous one at 60 and 72 h, but is otherwise very similar. This forecast lies near the middle of the guidance envelope.

Sam is expected to slowly weaken during the next several days as it moves over progressively cooler waters and into an environment of stronger shear. The hurricane will likely begin extratropical transition in a couple of days when it crosses the 26 degree C isotherm, and this transition is expected to be completed by 96 hours. All of the model guidance shows steady weakening through the forecast period, and so does the NHC forecast. This prediction is close to the IVCN, IVDR, and HCCA models in the short term and near the GFS model during the extratropical portion of the forecast. Even though Sam is likely to weaken, it is still expected to be a significant storm over the next several days.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next couple of days. Swells are expected to reach Bermuda and the Bahamas this afternoon, and then spread to the United States east coast and Atlantic Canada on Saturday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are possible on Bermuda beginning tonight or early Saturday, and a Tropical Storm Warning is in effect for the island.
FORECAST POSITIONS AND MAX WINDS

INIT  01/1500Z 28.4N  61.8W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  02/0000Z 30.7N  61.5W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  02/1200Z 33.4N  60.2W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 36H  03/0000Z 35.6N  58.4W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  03/1200Z 37.2N  56.0W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  04/0000Z 38.7N  52.9W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  04/1200Z 41.9N  48.6W   85 KT 100 MPH - Category 2 (ENE New York,NY)
 96H  05/1200Z 49.0N  42.2W   75 KT  85 MPH - Post Tropical (ENE St. John's,NF Canada)
120H  06/1200Z 51.0N  34.3W   60 KT  70 MPH - Post Tropical (ENE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 500 AM AST Fri Oct 01, 2021 

An Air Force Reserve Hurricane Hunter aircraft investigated Sam a few hours ago and found that the already-intense hurricane had strengthened a bit since yesterday afternoon’s reconnaissance flight. The plane measured a peak 700-mb flight-level wind of 147 kt in the northeastern eyewall, and using a typical 90-percent reduction from that altitude, the intensity is estimated to be 130 kt. Dropsondes released in the eye also indicated that the central pressure had fallen to 934 mb. This drop in pressure is likely due to a combination of the strengthening and an expansion of the tropical-storm- and hurricane-force wind fields, which was confirmed by the aircraft and scatterometer data from last evening.

Sam is moving a little faster toward the north-northwest (345/15 kt) as it curves around the western periphery of a mid-tropospheric ridge located over the eastern and central subtropical Atlantic. The hurricane is expected to turn northward later today and then northeastward by Saturday night–passing well to the east of Bermuda–when it begins to move between the ridge and a mid- to upper-level low located over the northeastern United States and Atlantic Canada. Interaction and a possible merger with the aforementioned low during extratropical transition is likely to cause a sudden north-northeastward acceleration to the southeast of Newfoundland on day 4, followed by a slower motion toward the east-northeast on day 5. The track models are very tightly clustered for much of the 5-day forecast period, yielding high confidence in the NHC track forecast. The new NHC forecast is relatively unchanged from the previous forecast, and closely follows the GFEX and TVCA consensus aids.

Although fluctuations in intensity will still be possible, a general weakening trend is anticipated in the coming days as Sam moves over a gradually cooler ocean, reaching sub-26C waters by day 3. However, the weakening is not expected to be rapid due to continued upper-level divergence and only light-to-moderate deep-layer shear, and Sam could remain a major hurricane at least through Saturday night. Global models now indicate that Sam should complete extratropical transition by day 4, and baroclinic forcing is likely to keep the system as a potent hurricane-force post-tropical cyclone through the end of the forecast period. The NHC intensity forecast is essentially an update of the previous forecast, lying close to the HCCA and IVCN consensus aids.

Key Messages:

  • 1. Swells generated by Sam will impact the northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next couple of days. Swells are expected to reach Bermuda and the Bahamas today, and then spread to the United States east coast and Atlantic Canada this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are expected on Bermuda beginning tonight or early Saturday, and a Tropical Storm Warning is in effect for the island.
FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 26.6N  61.7W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  01/1800Z 28.8N  62.0W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  02/0600Z 31.7N  61.2W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  02/1800Z 34.2N  59.7W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  03/0600Z 36.0N  57.8W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  03/1800Z 37.7N  55.0W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  04/0600Z 40.1N  51.4W   85 KT 100 MPH - Category 1 (ESE New York,NY)
 96H  05/0600Z 47.7N  44.8W   75 KT  85 MPH - Post Tropical (ENE St. John's,NF Canada)
120H  06/0600Z 50.1N  38.1W   65 KT  75 MPH - Post Tropical (ENE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 1100 PM AST Thu Sep 30, 2021

Sam’s satellite structure on infrared remains quite impressive this evening, with a circular eye staying quite warm (+10-20C) in comparison to the ring of cold cloud tops associated with the eyewall (-60 to -70C). The most recent microwave imagery was a SSMIS pass at 1957 UTC, which indicated the eyewall remains quite intense, if just a little asymmetric with a stronger western semicircle. This eyewall asymmetry is consistent with a touch of light southerly shear affecting the cyclone as diagnosed by ECMWF-SHIPS. However, this shear does not appear to be having a significant impact on the hurricane’s intensity. While there have not been any recent aircraft data to determine Sam’s intensity since the last mission departed the hurricane at around 2000 UTC, the satellite intensity estimates have not changed much this evening. Thus, the initial intensity has been held at 125-kt this advisory. Another Air Force Reserve Reconnaissance Mission will be flying into Sam overnight to get a better sense of the current intensity.

Following Sam’s eye tonight, the hurricane is beginning to make a gradual turn northward at a faster motion, with the latest estimate now north-northwest at 345/14 kt. The track guidance in the short-term remains in good agreement that Sam should continue to turn to the north, north-northeast, and then northeastward as it is steered around the western edge of a large subtropical ridge over the central Atlantic. This track should take the hurricane east of Bermuda. While there is a large deep-layer trough located north of Sam, this feature will not initially capture the cyclone, as this trough first de-amplifies and merges with a larger deep-layer trough located over the far eastern Provinces of Canada. In fact, Sam’s forward motion to the northeast in 48-72 hours may even slow a tad as it remains just south of the stronger upper-level westerlies. However, by 96-120 hours, the deep-layer low over Canada is expected to dig southeastward into North Atlantic, and will likely capture Sam as the hurricane rapidly accelerates ahead of this amplifying trough. It is in the 72-120 hour time-frame that a lot of the track guidance diverges, mostly related to the timing and degree of Sam’s interaction of with this trough ejecting out of Canada. For now, the official NHC track forecast has elected to stay close to the track consensus TVCN, which has preformed well with this hurricane so far. This latest track is just a bit northeast and faster than the previous one after 72 hours.

Sam still has another 24 hours under warm 28-29 C sea-surface temperatures while the southerly vertical wind shear is expected to remain light. Thus, the hurricane is expected to maintain Category 4 intensity over the next day or so, with short-term intensity changes likely to be controlled by inner core dynamics (such as eyewall replacement cycles). Thereafter, sea surface temperatures decrease markedly as southerly shear is also expected to gradually increase. This combination should lead to steady weakening. After 72 hours, the rate of weakening has actually been slowed a tad, since the ECMWF, GFS, and Canadian models all show Sam receiving significant baroclinic support as it undergoes a dynamic extratropical transition while the hurricane phases with the large baroclinic trough emerging off Canada. Simulated satellite imagery from these models suggest that Sam’s transition to an extratropical cyclone should take place between 96-120 hours, with the resulting cyclone producing a large wind footprint across the North Atlantic basin.

Key Messages:

  • 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by tomorrow, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning is in effect for the island.
FORECAST POSITIONS AND MAX WINDS

INIT  01/0300Z 25.0N  61.2W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  01/1200Z 27.2N  61.8W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  02/0000Z 30.2N  61.6W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  02/1200Z 32.9N  60.5W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  03/0000Z 35.0N  58.9W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 60H  03/1200Z 36.6N  56.8W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  04/0000Z 38.7N  53.8W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
 96H  05/0000Z 44.6N  47.2W   75 KT  85 MPH - Category 1 (ESE St.John's,NF Canada)
120H  06/0000Z 48.9N  42.5W   70 KT  80 MPH - Post Tropical (ESE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 500 PM AST Thu Sep 30, 2021 

Sam remains a formidable hurricane, with a well-organized cloud pattern on satellite imagery. The 25 n mi wide eye is surrounded by a fairly symmetric pattern of deep convective cloud tops, and the upper-level outflow pattern is very well defined. An Air Force Reserve Unit Hurricane Hunter aircraft investigated the system and found peak 700-mb flight level winds of 133 kt, and a highest unflagged SFMR-observed surface wind of 125 kt. Based on these observations, the advisory intensity is kept at 125 kt. Measurements from the aircraft also showed that the central pressure remains quite low, near 938 mb.

Sam will continue to traverse waters of high oceanic heat content for another 12-18 hours, and the shear should remain fairly low for the next few days. Therefore, some intensification could still occur overnight. In any event, the system is expected to maintain major hurricane status for the next 36-48 hours. Over the weekend, a gradual weakening trend should commence due to cooler SSTs. However, Sam will likely remain a powerful cyclone with hurricane- force winds until the end of the forecast period. By 120 hours, simulated satellite imagery from the ECMWF and GFS models forecast the system’s cloud pattern to resemble that of an extratropical low, and this is reflected in the official forecast, which shows extratropical status at that time. The official intensity forecast remains close to the HCCA corrected consensus model solution.

Center fixes continue to show a gradual increase in forward speed, and the hurricane is moving northwestward, or about 320/12 kt. During the next 24-36 hours, Sam should turn toward the north as it moves around the western periphery of a large subtropical high over the eastern and central Atlantic. Then, the cyclone is expected to move generally northeastward in the flow between the anticyclone and a trough near Atlantic Canada. There is fairly close agreement in the track models through 60 hours or so. However, there are some significant differences in the model-predicted track of Sam at higher latitudes, probably due to variations in how the system interacts with the trough in the various guidance. The NHC track forecast follows the latest model consensus, TVCN.

Although the core of Sam is predicted to pass east of Bermuda early Saturday, the tropical-storm-force wind field is forecast to expand and could pass very close to the island beginning late Friday night or early Saturday. Therefore, a Tropical Storm Warning has been issued for Bermuda.

Key Messages:

  • 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by Friday, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning has been issued for that island.
FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 23.6N  60.9W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  01/0600Z 25.6N  61.5W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  01/1800Z 28.5N  61.9W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  02/0600Z 31.3N  61.4W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 48H  02/1800Z 33.8N  60.1W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 60H  03/0600Z 35.8N  58.3W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  03/1800Z 37.4N  56.2W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
 96H  04/1800Z 42.5N  50.0W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)
120H  05/1800Z 47.5N  46.0W   65 KT  75 MPH - Post Tropical (ESE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 1100 AM AST Thu Sep 30, 2021

Sam continues to look quite impressive on satellite images, with a 20-25 n mi wide eye surrounded by very deep convection. The system’s convective banding features and upper-level anticyclonic outflow also remain well-defined, with particularly strong outflow to the northwest and north. Subjective and objective Dvorak intensity estimates have not changed significantly since earlier this morning, so the advisory intensity will remain at 125 kt. Interestingly, aircraft observations show that Sam has been a little stronger than indicated by satellite-derived intensity estimates, which underscores the value of aerial reconnaissance of tropical cyclones. Another Air Force Hurricane Hunter aircraft is scheduled to investigate the hurricane later today.

Sam should continue to move over waters of high oceanic heat content and within a low-shear atmospheric environment for the next 24 hours or so. Thus, it could still strengthen a little today and tonight. By 48 hours, decreasing SSTs and increasing shear should lead to a gradual weakening trend. Around day 5, global model fields suggest that Sam will be merging with a baroclinic zone and transition into a large and powerful extratropical cyclone over the north Atlantic. The official intensity forecast is in good agreement with the HCCA corrected consensus model prediction.

The hurricane is moving a little faster toward the northwest, or at about 320/11 kt. During the next couple of days, a gradual turn toward the north is likely as Sam rounds the western end of a large subtropical anticyclone over the eastern and central Atlantic. Thereafter, Sam is forecast to move generally northeastward in the flow between the anticyclone and a trough near Atlantic Canada. The NHC forecast is very similar to a consensus of the typically reliable GFS and ECMWF predictions, with a little bit of smoothing around days 3-4.

Although the core of Sam is forecast to pass east of Bermuda early Saturday, the tropical-storm-force wind field is forecast to expand and could pass very close to the island beginning late Friday night or early Saturday. Therefore, a Tropical Storm Watch is in effect for Bermuda.

Key Messages:

  • 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by Friday, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. Tropical storm conditions are possible on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Watch has been issued for that island.
FORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 22.6N  60.0W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  01/0000Z 24.4N  60.8W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  01/1200Z 27.2N  61.7W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  02/0000Z 30.2N  61.6W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  02/1200Z 32.7N  60.8W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
 60H  03/0000Z 34.9N  59.2W   95 KT 110 MPH - Category 2 (ENE St. George's,Bermuda)
 72H  03/1200Z 36.9N  57.0W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
 96H  04/1200Z 40.0N  51.0W   80 KT  90 MPH - Category 1 (ENE St. George's,Bermuda)
120H  05/1200Z 44.5N  45.5W   65 KT  75 MPH - Post Tropical (ESE St. John's,NF Canada)

NWS National Hurricane Center Miami FL 1100 PM AST Wed Sep 29, 2021 

Sam is intensifying tonight after completing an eyewall replacement cycle. A recent NOAA Hurricane Hunter aircraft showed peak 700 mb flight-level winds of 138 kt, multiple SFMR surface winds of 120 kt or greater, plus falling central pressure to about 940 mb. This deepening trend is also seen on recent satellite imagery, with significant warming within the eye noted. These data support raising the initial intensity to 125 kt for this advisory. It is of note that the NOAA mission has been quite helpful for both the size and intensity analysis, with dropsondes and SFMR data showing that the hurricane has grown quite a bit since earlier today, mostly on the eastern side. Additionally, reflectivity data from the core confirmed the end of the eyewall replacement cycle.

The hurricane could strengthen a little more overnight while it moves over a warm ocean eddy in fairly light shear. Afterward, gradually cooling SSTs, falling ocean heat content, and the inevitable future eyewall replacement cycles should cause a weakening trend. However, this will likely not be as steady as shown below, and significant deviations (upward or downward) from the forecast can be expected. Faster weakening is expected by the weekend due to cool SSTs and increasing shear. The new forecast is close to the intensity consensus, and is a little higher than the previous NHC prediction early on, primarily due to the initial wind speed.

Sam has sped up tonight, now estimated at 315/10 kt. It sounds like a broken record, but there are no changes to the forecast during the first few days as model guidance is in excellent agreement on a gradually rightward-curving track, passing a couple of hundred n mi or more east of Bermuda. Afterward, guidance is actually in worse agreement over the weekend, with widely divergent solutions from an upper-level trough picking up Sam and turning it north toward Newfoundland or the trough leaving Sam behind, causing a slow eastward motion. The trend in the guidance is for a slower motion at long range, so I’ve elected to keep the day 4 and 5 points almost the same as the previous advisory, just a hair faster than the latest consensus aids.

Key Messages:

  • 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
  • 2. A Tropical Storm Watch could be required for Bermuda on Thursday, and interests there should monitor the progress of Sam.
FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 20.6N  58.4W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  30/1200Z 21.9N  59.6W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  01/0000Z 24.1N  60.9W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  01/1200Z 26.8N  61.8W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  02/0000Z 29.6N  61.8W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  02/1200Z 32.2N  60.7W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 72H  03/0000Z 34.3N  59.2W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  04/0000Z 37.5N  54.5W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
120H  05/0000Z 42.0N  48.5W   70 KT  80 MPH - Category 1 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 500 PM AST Wed Sep 29, 2021 

Sam continues to exhibit a well-organized cloud pattern on satellite imagery, although there is some asymmetry of the coldest-topped convection around the center. The hurricane is also maintaining numerous banding features with well-defined upper-level anticyclonic outflow. Observations from an Air Force Hurricane Hunter plane showed peak 700 mb fight-level winds of 130 kt in the northeast quadrant which, using the standard reduction, corresponds to maximum surface winds of 117 kt. However, the maximum SFMR-observed surface winds were 103 kt. Based on these data, with more emphasis on the flight-level winds, the current intensity estimate is held at 115 kt. The aircraft also measured a minimum central pressure of 945 mb by dropsonde.

Since the hurricane is forecast to remain in an environment of low vertical shear and to remain over warm waters through 48 hours, Sam could restrengthen a little more during the next day or so. Since the Hurricane Hunters reported a concentric eyewall, short-term strengthening may be curtailed until the eyewall replacement cycle completes. Later in the forecast period, gradual weakening is likely due to cooler SSTs and increasing shear. The official intensity forecast is in very good agreement with the latest corrected consensus prediction. Sam will likely be close to extratropical transition around day 5.

The initial motion is still northwestward, or 310/8 kt. During the next few days, Sam is likely to move northwestward and then northward around the western periphery of a large subtropical high pressure system centered over the eastern Atlantic. A turn to the north-northeast and northeast is expected later in the forecast period as Sam begins to encounter the flow on the southern side of a large mid-tropospheric trough moving through Atlantic Canada. The track guidance models remain in excellent agreement through 72 hours, including the time that Sam passes by Bermuda. At days 4-5 there is some divergence of the model guidance, apparently due to how Sam may interact with the trough. Most notably the ECMWF is a lot slower than the other global models suggesting that the trough will at least partially bypass it in that case. Not much change has been made to the previous official forecast, however, and the NHC track remains very close to the TVCN dynamical model consensus.

Key Messages:

  • 1. Swells generated by Sam will impact the Lesser Antilles during the next several days. Swells are expected to reach Bermuda and the Bahamas in a day or so, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FORECAST POSITIONS AND MAX WINDS

INIT  29/2100Z 20.2N  57.6W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  30/0600Z 21.2N  58.9W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  30/1800Z 23.1N  60.4W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  01/0600Z 25.6N  61.4W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  01/1800Z 28.3N  61.9W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  02/0600Z 31.0N  61.3W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 72H  02/1800Z 33.6N  59.9W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  03/1800Z 37.5N  55.0W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)
120H  04/1800Z 42.0N  48.5W   70 KT  80 MPH - Category 1 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 AM AST Wed Sep 29, 2021 

Sam has a well-organized cloud pattern on satellite imagery, with a 90 n mi-wide Central Dense Overcast surrounded by numerous convective banding features. Upper-level outflow has now become better established to the southeast of the hurricane. A NOAA Hurricane Hunter mission from a couple of hours ago reported a 20 n mi-wide diameter circular eye. The current intensity is held at 115 kt for this advisory, which is a little higher than the maximum winds observed in the earlier aircraft mission. However, since the estimated minimum central pressure of 949 is a little lower than it was earlier this morning and Sam’s intensity has been fluctuating, it is considered prudent not to lower the winds at this time. An Air Force Hurricane Hunter aircraft is scheduled to investigate the hurricane soon and will provide another intensity estimate.

Sam continues its slow northwestward trek with the current motion remaining near 310/8 kt. Sam should move northwestward and then northward around the western portion of the subtropical ridge over the eastern Atlantic during the next few days, and turn toward the north-northeast and northeast later in the forecast period. The track guidance models are in very good agreement through 72 hours or so, with some increase in model spread thereafter. This increase in spread is largely due to how Sam will interact with a strong mid-latitude trough moving through Atlantic Canada. Some of the guidance suggests that this trough could partially bypass the hurricane around the end of the period and beyond, but this is not yet clear. The current NHC forecast track is not much different from the previous one and is in good agreement with the dynamical model consensus, TVCA.

The hurricane will continue to move over warm waters and in an environment of low vertical shear and diffluent upper-level flow for the next 1-2 days. Thus, it would not be surprising to soon see some restrengthening. The official intensity forecast is essentially a blend of the latest Decay-SHIPS and LGEM predictions, and is very similar to the previous NHC forecast. Sam is likely to remain a major hurricane into this weekend.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 19.4N  57.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  30/0000Z 20.3N  58.1W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  30/1200Z 21.8N  59.6W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  01/0000Z 23.9N  61.0W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  01/1200Z 26.6N  61.8W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  02/0000Z 29.4N  61.8W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 72H  02/1200Z 32.0N  60.9W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  03/1200Z 36.2N  57.3W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)
120H  04/1200Z 39.5N  53.0W   75 KT  85 MPH - Category 1 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 PM AST Tue Sep 28, 2021 

Satellite images suggest that Sam has strengthened. The eyewall of the hurricane has become more intense, with a warmer eye noted during the past several hours since the last reconnaissance aircraft departed. The initial wind speed is nudged upward to 120 kt, a little above what the previous aircraft mission from this afternoon supported. Another Air Force Reserve Hurricane Hunter plane is scheduled to fly through Sam in a few hours for a better look at the intensity.

Sam is forecast to move into an area with somewhat lower wind shear and higher ocean heat content by late Wednesday. In addition, the hurricane will likely be moving a little faster, which decreases the upwelling potential under the storm. These factors lead me to believe Sam isn’t quite done intensifying, and could reach another peak sometime late Wednesday or Thursday. The one fly-in-the- ointment to this forecast is another eyewall replacement cycle potentially beginning, although recent microwave data doesn’t give that impression. Thus, the new forecast is raised from the last one during the first couple of days, on the higher side of the guidance. A more consistent weakening trend is expected late week, due to cooler SSTs and increasing shear, and no significant changes were made to the official forecast at long range.

The hurricane continues moving northwestward at about 8 kt. Models are locked into this track continuing for the next day or two at a faster pace around the southwestern portion of a strengthening Atlantic subtropical ridge. Thereafter, Sam should turn northward on Friday and northeastward this weekend, likely a fair distance east of Bermuda, due to a large mid-latitude trough moving off of the United States east coast. Model guidance remains in close agreement on almost all of the forecast, and the only significant change from the last advisory is an eastward adjustment on Day 5 due to guidance suggesting that Sam gets shunted more eastward by the trough, rather than captured by it.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 18.4N  55.6W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  29/1200Z 19.1N  56.7W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  30/0000Z 20.2N  58.2W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  30/1200Z 21.8N  59.9W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  01/0000Z 23.9N  61.2W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  01/1200Z 26.4N  61.9W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  02/0000Z 29.1N  61.8W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  03/0000Z 35.0N  58.0W  100 KT 115 MPH - Category 3 (ENE St. George's,Bermuda)
120H  04/0000Z 40.0N  52.0W   85 KT 100 MPH - Category 2 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 500 PM AST Tue Sep 28, 2021

Sam has looked a little less impressive on satellite imagery this afternoon, particularly on the infrared channels where the eye is not very well-defined. However, observations from the Air Force Hurricane Hunters indicate that the hurricane is maintaining category 4 intensity since a blend of the latest flight-level and SFMR-observed surface winds support maintaining 115-kt winds. The aircraft also measured a quite low central pressure of 947 mb on two passes through the eye. Upper-level outflow remains well defined except over the southwest quadrant of the circulation, and several arc clouds are seen emanating from the system over the western semicircle, indicative of some dry mid-level air in the environment. However, this dry air is apparently not significantly affecting the inner core of Sam, given that it has remained a powerful hurricane.

Center fixes yield a continued slow northwestward motion of around 305/8 kt. There is little change to the track forecast reasoning from the previous advisories. Sam is likely to move around the southwestern periphery of a subtropical ridge for the next couple of days. A turn toward the north, ahead of a large mid-tropospheric trough moving off the United States east coast, is forecast around day 3. Later in the forecast period, the hurricane is expected to accelerate northeastward within the flow on the eastern side of the trough. There is good agreement among most of the guidance models on this general track, and the official forecast is very close to the model consensus.

Since Sam will remain in a low-shear environment and over a warm ocean surface for the next few days, some re-intensification is possible, and the system is likely to remain a major hurricane for the next 4 days or so. The official intensity forecast is not much different from the latest NOAA corrected consensus forecast, HCCA.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 17.9N  55.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  29/0600Z 18.6N  55.9W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  29/1800Z 19.7N  57.4W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  30/0600Z 21.0N  59.1W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  30/1800Z 22.8N  60.6W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 60H  01/0600Z 25.0N  61.6W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 72H  01/1800Z 27.7N  62.0W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  02/1800Z 33.5N  59.0W  100 KT 115 MPH - Category 3 (ENE St. George's,Bermuda)
120H  03/1800Z 40.0N  53.5W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 AM AST Tue Sep 28, 2021

atellite images indicate that Sam has been generally steady in strength during the past several hours, and it remains a powerful category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. The storm has a well-defined eye, and the convective pattern surrounding the center is fairly symmetric. A 1043 UTC SSMIS pass showed a closed and symmetric eyewall, with no indication of a concentric eyewall. The latest Dvorak classifications from TAFB and SAB were 5.5/102 kt and the CIMSS automated technique is currently 6.2/120 kt. Based on these satellite estimates and the overnight reconnaissance data, the initial intensity is held at 115 kt. Another Air Force Hurricane Hunter aircraft is scheduled to investigate Sam this afternoon.

The major hurricane is moving slowly northwestward, with the initial motion estimated to be 305/7 kt. A mid-level ridge situated to the north-northeast of Sam is expected to shift eastward as a deep-layer trough digs southeastward over the western Atlantic. This change in the large-scale pattern should cause Sam to speed up and gradually turn toward the north in 2 or 3 days and then to the northeast in 4 to 5 days. The models are in fairly good agreement, and the new forecast track is essentially an update of the previous one. Based on the forecast, Sam should pass well east of the northern Leeward Island through Wednesday and east of Bermuda Friday night and Saturday.

Sam is expected to remain in favorable environmental conditions for it to maintain its intensity, or perhaps strengthen slightly, during the next day or two. However, after that time, a gradual increase in southerly shear should promote a slow weakening trend, and Sam will likely be beginning its transition to an extratropical cyclone toward the end of the forecast period. The NHC intensity forecast is also an update of the previous one and in line with the various consensus aids. Sam is expected to remain a major hurricane during the next several days.

The initial wind radii have been adjusted based on a 1114 UTC ASCAT-A pass.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 17.5N  54.3W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  29/0000Z 18.2N  55.1W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  29/1200Z 19.2N  56.5W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  30/0000Z 20.5N  58.1W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  30/1200Z 22.0N  59.6W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 60H  01/0000Z 24.0N  60.9W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 72H  01/1200Z 26.6N  61.5W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  02/1200Z 32.5N  59.7W  100 KT 115 MPH - Category 3 (ENE St. George's,Bermuda)
120H  03/1200Z 39.2N  54.8W   90 KT 105 MPH - Category 2 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 PM AST Mon Sep 27, 2021 

The NOAA Hurricane Hunters have been flying through Sam tonight, with radar and wind observations showing broken concentric eyewalls at times. The maximum 700-mb flight-level winds on the mission were 114 kt, with peak SFMR values to 103 kt and a central pressure around 956 mb, so the initial wind speed will remain 105 kt. While the overall intensity doesn’t seem to have changed much in the past several hours, the aircraft data show that the hurricane has grown in size, and that is reflected in the initial wind radii.

The future intensity of Sam is hard to pinpoint. Eyewall cycles will likely play a key role in determining its second peak intensity since the hurricane should remain in low- or moderate-shear and warm-water environments for the next few days. Those cycles are basically impossible to forecast, but there is some suggestion on satellite that a larger eye is trying to form, which could allow for some increase in strength in the near term. It seems likely that Sam will be a category 3 or 4 hurricane given the conducive environment for most of the forecast period, so the new NHC forecast is similar to the previous one, with the most significant change showing a larger hurricane consistent with the latest guidance.

Sam continues to move northwestward, or 315/8 kt. The hurricane should move in that direction for the next few days around the southwestern portion of the subtropical high. A turn to the north is likely as Sam meets the western edge of the high, with a gradual acceleration to the northeast anticipated by the weekend due to an incoming mid-latitude trough from the west. While there are some speed differences in the models, especially near and after recurvature, the NHC forecast leans toward the faster GFS solution over the slower ECMWF, given the magnitude of the strong southwesterly flow that Sam should encounter at long range, plus a nod to the superior performance of late for the American model. The new forecast is adjusted a bit to the right and faster beyond day 3, with no significant changes made before that time.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 16.8N  53.2W  105 KT 120 MPH - Category 3 (ENE Long Bay,Barbados)
 12H  28/1200Z 17.5N  54.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  29/0000Z 18.4N  55.1W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  29/1200Z 19.3N  56.4W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 48H  30/0000Z 20.6N  58.0W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 60H  30/1200Z 22.2N  59.7W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 72H  01/0000Z 24.3N  61.0W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 96H  02/0000Z 29.6N  61.5W  105 KT 120 MPH - Category 3 (ESE St. George's,Bermuda)
120H  03/0000Z 37.5N  56.9W  100 KT 115 MPH - Category 3 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 500 PM AST Mon Sep 27, 2021

Sam’s weakening trend ended earlier today, with the intensity likely bottoming out between the issuance of the previous advisory and now. Over the past few hours, microwave data and satellite imagery indicate that the inner core and eyewall of the hurricane are making a comeback. There is now a clear eye in visible imagery, convective cloud tops colder than -65 degrees C wrap completely around the center, and the convective mass is becoming more circular once again. An Air Force Reserve Hurricane Hunter aircraft currently investigating Sam indicates the central pressure has dropped to 957 mb, with peak 700-mb flight-level winds of 106 kt, SFMR values of 100 kt, and dropsonde surface winds of 102 kt. Given possible minor undersampling and the recent dropping of the hurricane’s pressure, the initial advisory intensity is set at 105 kt.

Sam continues to move northwest, or 315/8 kt. The hurricane is expected to continue this northwestward motion around a subtropical ridge over the next few days. By late Thursday or early Friday a turn toward the north is anticipated as Sam rounds the western periphery of the ridge. The cyclone should begin to gradually accelerate during that time in the increasing flow ahead of a mid- to upper-level trough emerging off the U.S. east coast later this week. Model guidance remains in very good agreement on this scenario and the latest NHC forecast track is little changed from the previous one, and lies near tightly clustered consensus track guidance.

The southwesterly winds that brought in some dry air and disrupted Sam’s impressive satellite appearance last night appear to have abated. As long as environmental winds surrounding the hurricane remain light, then the dry air surrounding the cyclone should not have as much of an impact on the hurricane’s strength over the next few days. Based on the improving inner-core structure appearance of Sam, re-strengthening is forecast over the next 12 h. Thereafter, mainly minor fluctuations in intensity are indicated through 72 h. Beyond that time, increasing vertical wind shear and decreasing sea-surface temperatures should cause Sam to weaken. The latest NHC intensity forecast was nudged slightly upward through 36 h and is on the high end of the guidance. Thereafter, no changes were made, and that portion of the forecast is close to the various multimodel consensus solutions.

Key Messages:

  • 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend.
FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 16.3N  52.7W  105 KT 120 MPH - Category 3 (ENE Long Bay,Barbados)
 12H  28/0600Z 17.0N  53.5W  115 KT 130 MPH - Category 4 (ENE St. John’s,A&B)
 24H  28/1800Z 17.8N  54.6W  115 KT 130 MPH - Category 4 (ENE St. John’s,A&B)
 36H  29/0600Z 18.8N  55.8W  115 KT 130 MPH - Category 4 (ENE St. John’s,A&B)
 48H  29/1800Z 19.8N  57.3W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 60H  30/0600Z 21.2N  59.0W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 72H  30/1800Z 22.9N  60.5W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
 96H  01/1800Z 27.8N  62.0W  105 KT 120 MPH - Category 3 (ENE St. George's,Bermuda)
120H  02/1800Z 34.7N  59.6W  100 KT 115 MPH - Category 3 (ENE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 AM AST Mon Sep 27, 2021

Sam appeared weaker on satellite images earlier this morning, and the eye was not apparent in visible or infrared satellite imagery. However, a 27/0741 UTC SSMIS pass still showed a well-defined eye. Recent visible and infrared images from the past couple of hours however, show that the structure is becoming better organized again, with the eye again becoming apparent on visible satellite. A NOAA Hurricane Hunter aircraft has made one pass through the hurricane so far and found that the central pressure has risen to about 966 mb. The current intensity estimate is reduced slightly to 110 kt. The aircraft has not yet sampled the northeast quadrant, where the strongest winds are likely occurring.

The initial motion is estimated to be northwestward, or 310/7. A subtropical ridge to the north and northeast of Sam will remain the primary steering mechanism over the next several days, and Sam is expected to continue on a similar heading through the next 72 hours or so. A mid- to upper-level trough is forecast to emerge off the U.S. Northeast and Mid-Atlantic coast and dig southward over the western Atlantic late this week. The deep-layer southerly flow ahead of this feature should cause Sam to accelerate to the north-northwest by late Thursday and then turn northward on Friday. The NHC forecast track is largely unchanged from the previous, except it is shifted ever so slightly westward at the hour 72 and 96 points, about midway between the previous NHC forecast and the latest HCCA consensus. The confidence in the track forecast is medium to high through 3 to 4 days. Beyond that time, confidence is about average, as there is some increase in model spread noted.

Regarding the intensity forecast, Sam is forecast to travel over warm ocean temperatures while it remains in an environment of low vertical wind shear through the next 4 days. Some mid-level dry air noted on water vapor channels to the west of Sam could play a role in limiting Sam’s intensity through hour 48. However, the environment could become more moist again after that time. Due to these competing factors, I opted to hold the intensity steady for the next several days, although fluctuations in intensity can be expected. The NHC intensity forecast is above all model guidance through hour 60, and is in agreement with the model consensus at hour 72-96. After that time, increasing wind shear induced by the approaching mid- to upper-level trough to the northwest of the cyclone should cause weakening.

FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 15.7N  52.1W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 12H  28/0000Z 16.4N  52.8W  110 KT 125 MPH - Category 3 (ENE La Désirade,Guadeloupe)
 24H  28/1200Z 17.3N  53.9W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 36H  29/0000Z 18.1N  55.1W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 48H  29/1200Z 19.1N  56.5W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 60H  30/0000Z 20.2N  58.1W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 72H  30/1200Z 21.7N  59.8W  110 KT 125 MPH - Category 3 (ENE St. John’s,A&B)
 96H  01/1200Z 25.9N  62.2W  110 KT 125 MPH - Category 3 (ESE St. George's,Bermuda)
120H  02/1200Z 32.0N  61.3W  100 KT 115 MPH - Category 3 (ESE St. George's,Bermuda)

NWS National Hurricane Center Miami FL 1100 PM AST Sun Sep 26, 2021 

A NOAA Hurricane Hunter aircraft investigating Sam this afternoon and evening found that the major hurricane likely peaked in intensity at around 135 kt with a central pressure of about 929 mb between 1900-2200 UTC when the eye contracted down to about 7 nmi in diameter. Since that time, however, wind and radar data from the aircraft reveal that Sam has been undergoing an eyewall replacement cycle (ERC), including significant erosion of the eyewall convection in the eastern semicircle, which has resulted in the central pressure increasing by at least 14 mb in only a few hours. The eye diameter has also doubled in size now. The ERC is also confirmed by the ragged inner-core convection and cloud-filled eye that have developed in satellite imagery. The advisory intensity of 125 kt is based on a blend of the available reliable SFMR surface wind data and flight-level wind data, and also using the pressure-wind relationship for a 943-mb central pressure.

The initial motion estimate is now 315/06 kt. A strong subtropical ridge to the north and northeast of Sam is forecast by the global and regional models to remain entrenched across the central and eastern Atlantic for the next several days, resulting in the hurricane moving northwestward for the 72 hours. Thereafter, a mid- to upper-level trough is forecast to dig southward just off the U.S. east coast, creating a deep southerly flow regime that will act to accelerate Sam northward on days 4 and 5. The new NHC track forecast is similar the previous advisory, and lies close to the tightly packed consensus models through 72 hours. In the 96-120 hour period, the models diverge significantly on when and where Sam will begin moving out toward the north, and the NHC track forecast favors the more eastward GFS solution since that model has been performing quite well with Sam thus far.

Now that Sam is undergoing an ERC, fluctuations in intensity could occur for the next day or two while the vertical wind shear remains low at least than 10 kt. Thereafter, a slow weakening trend is expected due to a modest increase in the wind shear from the southwest and occasional intrusions of drier mid-level air. The NHC intensity forecast is a little lower than the previous advisory, mainly due to the lower initial intensity, and closely follows the HCCA and IVCN consensus model intensity forecasts.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0300Z 14.7N  50.8W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  27/1200Z 15.3N  51.6W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  28/0000Z 16.2N  52.8W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  28/1200Z 17.0N  53.9W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  29/0000Z 17.8N  55.0W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  29/1200Z 18.7N  56.3W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  30/0000Z 20.0N  58.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  01/0000Z 23.4N  61.0W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  02/0000Z 28.4N  62.6W  105 KT 120 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 500 PM AST Sun Sep 26, 2021 

Sam has found a way to strengthen some more today. The ring of intense convection surrounding the 15-n-mi-diameter eye has expanded in size over the past several hours, while cloud tops colder than -70C still completely surround the center. An earlier GMI microwave overpass revealed very strong eyewall convection tightly wrapped around the small eye, a feature oftentimes seen with intense hurricanes. The latest Dvorak intensity estimates from TAFB and SAB remain T6.5/127 kt. However, the UW-CIMSS ADT estimate has recently increased to T6.6/130 kt. Based on the expansion of the ring of convection and a blend of these Dvorak estimates, the initial intensity is raised to 130 kt for this advisory. Sam remains compact, with winds of tropical storm force extending only 70 n mi from the center. A NOAA Hurricane Hunter aircraft is en route to investigate Sam and will provide more detailed intensity data.

It would not take much further expansion of the convection and cooling of the cloud tops over the inner-most core of Sam for it to become a rare Category 5 hurricane on the Saffir-Simpson hurricane wind scale. Although plausible given the ideal environmental conditions over the next couple of days, this strengthening is not explicitly forecast to occur. The only factors within the next 2-3 days that could cause Sam to weaken would be an eyewall replacement cycle or the upwelling of cooler waters due to the hurricane’s relatively slow forward motion. By 72 h, vertical wind shear is forecast to increase, which should cause gradual weakening. However, given the current strength of Sam, the new forecast calls for it to remain a major hurricane throughout the 5-day forecast period. The latest NHC intensity forecast was increased by 5 kt through 60 h due to the change in the initial intensity and remains on the high end of the guidance through that time. At 72 h and beyond, the intensity forecast is near the HFIP corrected consensus approach (HCCA) model.

Sam has been wobbling a little north-northwestward and has slowed down over the past few hours, but the 12-h-average motion is about 305/06 kt. A subtropical ridge centered to the northeast of the hurricane should steer it northwestward for the next 3-4 days. By late Thursday, Sam is expected to reach the western periphery of the ridge and turn north-northwestward and begin to increase its forward speed as the cyclone starts to get caught up in the southerly flow between the ridge and an approaching mid-latitude trough. The various track model guidance is in better agreement today compared to yesterday, especially in the day 3-5 time frame. The latest NHC track forecast is little changed from the previous one, and lies near the various consensus track model solutions.

FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 14.2N  50.5W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  27/0600Z 14.7N  51.3W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  27/1800Z 15.5N  52.5W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  28/0600Z 16.3N  53.6W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  28/1800Z 17.2N  54.7W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  29/0600Z 18.1N  55.8W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  29/1800Z 19.2N  57.4W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  30/1800Z 22.1N  60.5W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  01/1800Z 26.6N  62.9W  110 KT 125 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 1100 AM AST Sun Sep 26, 2021 

The satellite presentation of Hurricane Sam has been quite steady over the past 6 hours. Sam continues to have a well-defined, 12 n mi wide eye on visible and infrared satellite imagery this morning, with cloud tops colder than -70C completely surrounding the center. The latest subjective satellite intensity estimates from TAFB and SAB were both T-6.5/127 kt, while the objective estimate from ADT is near 122 kt. Based on these data, there is no reason to change the 125-kt initial intensity at this time, especially given that a NOAA Hurricane Hunter aircraft is scheduled for a late afternoon/early evening mission into Sam today.

Sam is moving toward the west-northwest, or 290/7 kt. A deep-layer subtropical ridge to the north and northeast of Sam should generally steer the hurricane west-northwestward to northwestward for the next several days. By midweek, an amplifying upper-level trough will move off the east coast of the U.S. and dig southward over the western Atlantic, which will erode the western extent of the steering ridge. The deep-layer southerly flow ahead of the trough should cause Sam to begin recurving, with a somewhat faster north-northwestward motion forecast by day 5. The latest NHC track forecast was adjusted a little to the left, or southwest of the previous official forecast, mainly at days 4 and 5, but not quite as far southwest as the model consensus aids.

The hurricane will likely experience some fluctuations in intensity during the next few days. Guidance, however, indicates that the chance of an eyewall replacement cycle during the next 36 hours is below climatology. Sam will remain over warm sea-surface temperatures and in a low wind shear environment for the next 3 days, with less than 10 kt of shear diagnosed by the SHIPS guidance. It is possible that Sam’s slow forward motion during the next 2 to 3 days could cause some upwelling of cooler water, but this will probably not result in significant weakening. The official NHC intensity forecast lies on the high end of the guidance for the first 36-48 h of the forecast period. Thereafter, the NHC forecast is closer to the middle of the guidance envelope. Some gradual weakening is forecast later in the period as southwesterly vertical wind shear begins to increase, but Sam is still forecast to remain a major hurricane through the 5-day period.

The radii were expanded slightly by 5-10 n mi in the western semicircle from the previous advisory based on a 1155 UTC ASCAT-A pass.

FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 13.9N  50.2W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  27/0000Z 14.4N  51.2W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  27/1200Z 15.2N  52.4W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  28/0000Z 16.0N  53.5W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  28/1200Z 16.8N  54.6W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  29/0000Z 17.7N  55.7W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  29/1200Z 18.6N  57.1W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  30/1200Z 21.4N  60.3W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  01/1200Z 25.6N  63.1W  110 KT 125 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 1100 PM AST Sat Sep 25, 2021

It’s been an interesting evening with regards to analyzing the various data from a NOAA Hurricane Hunter research flight into Hurricane Sam. Dropsonde data in both the southeast and northwest quadrants indicate that small eyewall mesovortices and possibly even tornado-scale vortices were present based on the wind profiles showing sharply opposite-direction winds from what would normally be expected in those regions of the hurricane. Some dropsonde surface winds have been as high as 162 kt, which is more representative of a gust, while SFMR surface winds have been as high as 133 kt. However, the strongest 700-mb flight-level winds have been 138-139 kt in the northeastern quadrant, which equates to about 125-kt tangential surface winds. Three dropsondes released in the eye indicate that the pressure had remained steady at 943-944 mb during the duration of the aircraft reconnoiter. That pressure equates to about 125 kt based on various pressure-wind relationships. Based on that estimate and the 700-mb flight-level to surface-wind conversion, the advisory intensity is 125 kt, which is representative of the mean tangential winds and no localized wind perturbations.

The initial motion is toward the west-northwest, or 295/07 kt. Not to sound like a broken record, but no significant changes were made to the previous track forecast and reasoning. Sam is expected to move slowly west-northwestward and northwestward over the next few days around the southwestern periphery of a deep-layer subtropical ridge that is situated to the north and northeast of the small hurricane. On days 4 and 5, an upper-level trough/low is forecast to dig southward and amplify off the U.S. east coast and extend all the way to the Bahamas. This feature should act to lift Sam northward at a faster forward speed. The latest NHC model guidance based on 12Z and 18Z model runs has shifted noticeably to the east of the previous runs, and the new NHC track forecast has been nudged in that direction as well. However, since the NOAA G-IV jet aircraft has been out there sampling the environment around Sam, it’s best to remain conservative and not shift the track any farther to the east until the new 00Z model runs with that new aircraft data come in for the next advisory package at 0600Z. The new NHC track forecast lies about halfway between the previous advisory track on the left and the tightly packed consensus track models on the right.

The radar images from the reconnaissance aircraft indicated that the eyewall was thin in many locations due to dry-air intrusions, and the latest SHIPS intensity output indicates that Sam will remain within a fairly dry mid-level environment. Also, the depth of the warm water beneath the hurricane isn’t overly deep, which could result in cold upwelling owing to Sam’s slow forward motion of only 5-7 kt during the next couple of days. Eyewall replacement cycles are also likely now due to the hurricane’s small size and strong intensity. Thus, fluctuations in intensity seem likely for the next couple of days even though the vertical wind shear is expected to remain quite low at only 5-10 kt. On days 3-5, however, the shear is forecast to increase to 15-20 kt from the southwest, which is expected to induce a slow weakening trend. However, it is likely that Sam will remain a major hurricane through 120 h, even on days 3-5 due to the cyclone moving over warmer and deeper water during that 3-day period. The new official intensity forecast is essentially the same as the previous advisory, and remains above the consensus model and is near the higher end of the intensity guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0300Z 13.5N  49.0W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  26/1200Z 14.0N  49.9W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  27/0000Z 14.7N  51.0W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  27/1200Z 15.4N  52.1W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  28/0000Z 16.3N  53.2W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  28/1200Z 17.1N  54.3W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  29/0000Z 18.0N  55.6W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  30/0000Z 20.1N  58.5W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  01/0000Z 23.7N  61.4W  110 KT 125 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 500 PM AST Sat Sep 25, 2021

After a brief hiccup, Sam has resumed rapid strengthening and now shows a 12-15 n mi wide well-defined eye embedded in a central dense overcast with cloud tops colder than -70C. Subjective satellite intensity estimates from TAFB and SAB were 115 kt at 18Z, and objective intensity estimates have also been near 115 kt. Since the organization has increased some since 18Z, the initial intensity is increased to 120 kt. A NOAA Hurricane Hunter aircraft is currently enroute to Sam to provide a better estimate of the intensity. The hurricane has good cirrus outflow in all quadrants. However, a large arc cloud to the northwest of the central core suggests dry air is present in that area.

The initial motion is still west-northwestward or 290/9. There are again no changes to the forecast track reasoning. The weak subtropical ridge currently to the north of Sam is forecast to shift eastward as a large deep-layer mid-latitude trough develops over the western Atlantic. This evolution should cause Sam to continue west-northwestward with some decrease in forward speed during the next couple of days, followed by a turn toward the northwest. Some increase in forward speed is likely by 120 h as Sam encounters stronger steering flow. The track guidance has shifted a little to the north and east since the previous advisory, and the new forecast track is also nudged in that direction, with the new track lying between the HCCA corrected consensus model and the other consensus models. Based on the NHC forecast, and all of the other guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5.

The current strengthening is expected to continue for the next 6-12 h, and based on this the new intensity forecast now calls for a peak intensity of 130 kt. After the peak, Sam is expected to be in an environment of warm sea surface temperatures and light wind shear for at least the next 3-4 days. This suggests the intensity will be controlled mainly by eyewall replacement cycles and possible intrusions of dry air, which would cause some short-term fluctuations in intensity. In the bigger picture, the intensity guidance calls for a slow weakening during the 24-120 h forecast period, and the new intensity forecast does the same. However, it is likely that Sam will remain a major hurricane through 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 13.3N  48.5W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 12H  26/0600Z 13.7N  49.4W  130 KT 150 MPH - Category 4 (ENE Long Bay,Barbados)
 24H  26/1800Z 14.3N  50.5W  125 KT 145 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  27/0600Z 14.9N  51.6W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  27/1800Z 15.7N  52.7W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  28/0600Z 16.6N  53.9W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  28/1800Z 17.4N  55.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  29/1800Z 19.5N  58.0W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  30/1800Z 22.5N  61.0W  110 KT 125 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 1100 AM AST Sat Sep 25, 2021 

Sam has continued to intensify this morning and now has a complete ring of deep convection with cloud tops colder than -70 degrees C surrounding a mostly clear, small eye. The latest subjective Dvorak intensity estimates from TAFB and SAB are both 102 kt, while the combined objective Dvorak intensity estimates from the UW-CIMSS SATCON and ADT is 110 kt. Based on this data, the initial intensity has been increased to 105 kt for this advisory, making Sam a major hurricane. The cyclone remains compact, with hurricane-force winds extending only 20 n mi from the center.

The hurricane wobbled to the west over the past few hours. However, the 12-hour motion is west-northwest at 9 kt. There are no changes to the forecast track reasoning. The ridge the north of Sam responsible for its current motion is forecast to shift to the east in a couple of days as a deep-layer trough establishes itself over the western Atlantic. Sam is forecast to turn northwestward around the southwestern periphery of the ridge in about 48 h, and gradually increase its forward speed thereafter. The model guidance continues to be in very good agreement with this scenario, and the latest NHC track forecast is unchanged from the previous one. It should be noted that the westernmost guidance continues to be the ECMWF ensemble mean, whose members have had a low bias in the intensity of Sam, which is likely contributing to the westward track bias. Based on the NHC forecast, and all of the other guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5.

Environmental conditions support further strengthening in the short term, and Sam is expected to become a category 4 hurricane by Sunday. In a few days, some southwesterly shear is forecast to develop as the cyclone begins to round the periphery of the subtropical ridge. Although this should cause Sam to weaken, it is still expected to remain a powerful hurricane throughout the 5-day forecast period. The latest NHC intensity forecast is near the previous one, which remains slightly above the various consensus solutions through 72 h, and then closely follows the consensus thereafter.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 12.9N  47.6W  105 KT 120 MPH - Category 3 (ENE Long Bay,Barbados)
 12H  26/0000Z 13.3N  48.7W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 24H  26/1200Z 13.7N  49.8W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 36H  27/0000Z 14.3N  50.8W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  27/1200Z 15.0N  51.9W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  28/0000Z 15.8N  53.0W  115 KT 130 MPH - Category 3 (ENE Long Bay,Barbados)
 72H  28/1200Z 16.6N  54.1W  115 KT 130 MPH - Category 3 (ENE Long Bay,Barbados)
 96H  29/1200Z 18.4N  56.5W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  30/1200Z 20.7N  59.8W  110 KT 125 MPH - Category 3 (Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 1100 PM AST Fri Sep 24, 2021

Sam developed a 10-n mi wide eye in infrared satellite images a few hours ago, surrounded by a ring of cloud tops as cold as about -70 degrees Celsius. The core of the hurricane remains compact. In fact, recent Synthetic Aperture Radar (SAR) data indicated that the radius of maximum winds is only about 6 n mi. Dvorak estimates were T5.0/90 kt from TAFB and T4.5/77 kt from SAB at 0000 UTC, and given the additional clearing of the eye since then, Sam’s intensity is raised to 85 kt.

Sam has been moving toward the west-northwest during the past 6 to 12 hours with a motion of 285/12 kt. The track forecast reasoning remains the same, and the model guidance is packed more tightly than normal through day 5. A blocking ridge lying to the north and northwest is expected to cause Sam to slow down over the weekend and maintain a motion toward the west or west-northwest. On days 3 through 5, the ridge is expected to shift eastward, with a deep-layer trough developing over the western Atlantic. This should allow Sam to turn northwestward toward the end of the forecast period. The NHC track forecast has not budged much and remains close to the TVCA consensus aid. Based on this forecast, and nearly all of the reliable guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5.

Deep-layer shear is expected to be less than 10 kt for the next day or two, and Sam will be heading toward waters of higher oceanic heat content. These conditions favor continued strengthening, and the hurricane is forecast to become a major hurricane on Saturday and possibly reach a peak intensity in 2 to 3 days. For this period, the NHC forecast is close to the HCCA consensus aid and is near the high end of the guidance envelope. As is usually the case with hurricanes this small and strong, fluctuations in intensity due to eyewall replacement cycles will be possible. On days 3 through 5, some increase in shear as Sam approaches the deep-layer western Atlantic trough is possible, and modest weakening is shown at those times.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 12.4N  45.9W   85 KT 100 MPH - Category 2 (ESE Long Bay,Barbados)
 12H  25/1200Z 12.7N  47.4W  100 KT 115 MPH - Category 3 (ENE Long Bay,Barbados)
 24H  26/0000Z 13.1N  48.8W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 36H  26/1200Z 13.5N  50.0W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  27/0000Z 14.1N  51.2W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  27/1200Z 14.8N  52.4W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  28/0000Z 15.7N  53.5W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  29/0000Z 17.4N  55.8W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  30/0000Z 19.5N  58.8W  110 KT 125 MPH - Category 3 (ENE Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 500 PM AST Fri Sep 24 2021 

Sam’s structure on satellite has improved this afternoon, with overshooting cloud tops rotating quickly around the small central dense overcast, providing the appearance that the hurricane is mixing out the dry air that affected its core structure this morning. In fact, the last few visible satellite images suggest that a small eye is clearing out within the cirrus canopy. Interestingly, a recent 1930 UTC SSMIS pass suggests concentric bands are also forming in Sam’s core structure and it remains to be seen how that structure will affect the wind field evolution of the hurricane. The latest subjective Dvorak intensities estimates form TAFB and SAB were a consensus T4.5/75 kt, while the latest SATCON and ADT estimates from UW-CIMSS have also increased this afternoon. The current advisory intensity has been set to 75-kt. Given the recent trends on satellite, this may be conservative.

The motion of Sam has remained on the same heading, though the storm is beginning to slow down with the estimate now at 280/10 kt. An additional slowdown in forward motion is anticipated in the short-term as the mid-level ridging shuffles to the northwest of the tropical cyclone, impeding its forward motion. However, after 72 hours, this same ridge is expected to shift back to the northeast of Sam as a deep-layer trough off the Eastern United States coastline becomes established. Track guidance spread increases towards the end of the forecast, with the ECMWF and GFS remaining at odds, with the former on the equatorward side, and the latter on the poleward side of the track envelope. The consensus aids, however, have changed little from the previous advisory and the latest NHC track forecast is quite similar to the previous one.

Now that Sam appears to have mixed out the dry air seen this morning, intensification, likely rapid, is resuming. The only fly in the ointment is the current concentric banding structure on microwave imagery, which hints at the possibility of an eyewall replacement cycle. Assuming the smaller eye will not collapse in the short term, rapid intensification appears likely over the next 12-24 hours and the intensity has been raised in the short term, taking Sam to major hurricane intensity by tomorrow. The latest NHC intensity forecast now shows a slightly higher peak at 120-kt in 48 hours followed by very gradual decay, due to a subtle increase in vertical wind shear, potentially some upwelling effects due to the slow forward notion, and eyewall replacement cycles that could lead to a broadening of the wind field. The latest NHC intensity forecast starts out on the high side of the intensity guidance but ends up very close to the HCCA consensus aid by the end of the forecast period. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 12.1N  44.8W   75 KT  85 MPH - Category 1 (ESE Long Bay,Barbados)
 12H  25/0600Z 12.3N  46.5W   90 KT 105 MPH - Category 2 (ESE Long Bay,Barbados)
 24H  25/1800Z 12.7N  48.1W  105 KT 120 MPH - Category 3 (ENE Long Bay,Barbados)
 36H  26/0600Z 13.1N  49.3W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 48H  26/1800Z 13.5N  50.5W  120 KT 140 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  27/0600Z 14.3N  51.7W  115 KT 130 MPH - Category 3 (ENE Long Bay,Barbados)
 72H  27/1800Z 15.1N  52.9W  115 KT 130 MPH - Category 3 (ENE Long Bay,Barbados)
 96H  28/1800Z 16.9N  55.1W  110 KT 125 MPH - Category 3 (ENE Penville,Dominica)
120H  29/1800Z 18.9N  58.1W  110 KT 125 MPH - Category 3 (ENE Philipsburg,Saint Martin)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Sep 24, 2021 

After rapidly intensifying from a tropical depression to a hurricane over the past 24 hours, Sam is taking a momentary pause from intensification. The structure on visible satellite imagery remains quite healthy, with tightly coiled outer bands and a small but very cold central dense overcast. However, microwave passes at 0957 UTC and 1211 UTC on the 89-91 GHz channels show an erosion of the inner-core structure on the west side, which could be related to the dry-air slots mentioned in the previous advisory entraining into the core, temporarily disrupting the structure underneath the cirrus. The latest subjective Dvorak intensity estimates from TAFB and SAB were T4.0/4.0-65 kt and T4.0/4.5-75 kt while the latest UW-CIMSS ADT and SATCON objective estimates were both at 59 kt. The current intensity is being maintained at 65 kt for this advisory, taking a blend of these subjective and objective estimates. Sam remains a very small tropical cyclone, with the latest scatterometer wind data indicating that tropical-storm and hurricane-force winds have have not expanded much from last night.

Sam continues to move just north of due west, but is beginning to gradually slow down at 280/12 kt. Over the next 24-36 hours, Sam is expected to maintain a general west to west-northwest heading but will slow down gradually as the strongest mid-level ridging becomes oriented more to the northwest of the hurricane. This ridge repositioning is thanks in part to a strong deep-layer trough well northeast of Sam that is digging in south of the Azores. Towards the end of the forecast, this trough will cutoff and gradually decay, allow the mid-level ridging to build-in back east as another deep-layer trough becomes established just off the eastern United States coastline. In general, the track guidance is in fairly good agreement on this general solution. However, differences remain in the details between how strong the ridge will be immediately to the north of Sam at the end of the 5-day forecast. The deterministic GFS and Canadian models show weaker ridging that erodes sooner, allowing a bit more poleward motion, while the ECMWF has stronger ridging that places its track on the equatorward side of the guidance envelope. The consensus track aid TCVN blends the aforementioned model guidance and has changed little this cycle, and the latest NHC track forecast is largely an update of the previous advisory.

While the shear remains very low over Sam as diagnosed by the GFS- and ECMWF-based SHIPS guidance, the 700-500 mb layer mean relative humidity is a bit on the low side, and could possibly explain why Sam was susceptible to the dry-air intrusion that disrupted the core structure this morning. However, the vertical wind shear is expected to remain very low for the next 48-60 hours, and the hurricane should be able to easily mix out the dry air over the inner-core. The wind field also remains very compact, so once the inner-core closes back off, rapid intensification is expected to resume. The latest NHC intensity forecast still shows Sam becoming a major hurricane by tomorrow and currently has a peak intensity as a category 4 hurricane in 48-72 hours. Thereafter, a bit more vertical wind shear, and the likelihood of eyewall replacement cycles are likely to lead to fluctuations in the intensity that are difficult to predict ahead of time. The current intensity forecast remains close to the HCCA and IVCN consensus guidance. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 11.8N  43.7W   65 KT  75 MPH - Category 1 (ESE Long Bay,Barbados)
 12H  25/0000Z 12.0N  45.4W   75 KT  85 MPH - Category 1 (ESE Long Bay,Barbados)
 24H  25/1200Z 12.3N  47.1W   90 KT 105 MPH - Category 2 (ESE Long Bay,Barbados)
 36H  26/0000Z 12.7N  48.4W  105 KT 120 MPH - Category 3 (ENE Long Bay,Barbados)
 48H  26/1200Z 13.1N  49.7W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 60H  27/0000Z 13.7N  50.9W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 72H  27/1200Z 14.6N  52.1W  115 KT 130 MPH - Category 4 (ENE Long Bay,Barbados)
 96H  28/1200Z 16.4N  54.8W  110 KT 125 MPH - Category 3 (ENE La Désirade,Guadeloupe)
120H  29/1200Z 18.2N  57.7W  110 KT 125 MPH - Category 3 (ENE Codrington,Barbuda)

NWS National Hurricane Center Miami FL 1100 PM AST Thu Sep 23, 2021

Microwave data from around the time of the previous advisory revealed that Sam has developed a well-defined inner core, including a nearly closed eyewall feature in the 91-GHz channel. Dvorak estimates from TAFB and SAB have increased to T3.5 and T4.0, respectively, and the initial intensity is therefore estimated to be 60 kt. Sam’s 24-hour intensity change qualifies as rapid intensification, since the cyclone was only a 30-kt tropical depression at this time yesterday. Sam is small, however. Recent scatterometer data showed that tropical-storm-force winds only extend a maximum of 40 n mi from the center, and the radius of maximum winds is estimated to be about 15 n mi. This small size is likely to have implications on Sam’s future intensity.

Sam is moving a little slower toward the west (280/13 kt). A mid-tropospheric ridge is expected to build across the Caribbean Sea and extend northeastward over the central Atlantic during the next few days. This blocking high is expected to cause Sam to slow down to 6 to 7 kt by late Saturday and Sunday while it maintains a westward to west-northwestward heading. Global models suggest the ridge may weaken and shift eastward a bit by days 4 and 5, allowing Sam to turn to the northwest and speed up a little by the end of the forecast period. The ECMWF, HWRF, and HCCA models lie along the southern edge of the main pack of models, while the ECENS ensemble mean is even farther south. The GFS and the HMON are on the northern side. Partially due to an adjustment of the initial position, the new NHC track forecast has been shifted southward a bit from the previous forecast, and it lies just south of the TVCA consensus aid–but not as far to the left as the previously mentioned southern models.

Environmental conditions and Sam’s structure and size are ideal for continued rapid intensification (RI). The SHIPS RI index for a 30-kt increase in 24 hours is nearly 50 percent. The DTOPS methodology, which uses inputs from the statistical-dynamical and deterministic models, shows RI indices for various forecast periods exceeding 90 percent. Therefore, rapid strengthening is being forecast at least for the next 36 hours, with Sam becoming a hurricane very soon and then a major hurricane by Friday night or Saturday morning. One caveat to this forecast is that although Sam’s small size more readily allows for RI to occur, it can also make the cyclone more prone to weakening if, for example, vertical shear increases. By 48 hours, the intensity models show the strengthening trend leveling off, and the NHC intensity forecast follows suit. Fluctuations in intensity from days 3 through 5 are likely, both due to normal internal dynamics within the storm and the cyclone’s response to the surrounding environment in relation to its small size.

FORECAST POSITIONS AND MAX WINDS

INIT  24/0300Z 11.2N  40.9W   60 KT  70 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 12H  24/1200Z 11.4N  42.9W   75 KT  85 MPH - Category 1 (ESE Long Bay,Barbados)
 24H  25/0000Z 11.8N  45.0W   90 KT 105 MPH - Category 2 (ESE Grand'Rivière,Martinique)
 36H  25/1200Z 12.1N  46.6W  105 KT 120 MPH - Category 3 (ESE La Désirade,Guadeloupe)
 48H  26/0000Z 12.5N  47.8W  110 KT 125 MPH - Category 3 (ESE La Désirade,Guadeloupe)
 60H  26/1200Z 13.0N  49.1W  110 KT 125 MPH - Category 3 (ESE Long Bay,Barbados)
 72H  27/0000Z 13.5N  50.4W  110 KT 125 MPH - Category 3 (ENE Long Bay,Barbados)
 96H  28/0000Z 15.3N  53.2W  110 KT 125 MPH - Category 3 (ESE Penville,Dominica)
120H  29/0000Z 17.3N  56.1W  110 KT 125 MPH - Category 3 (ESE Codrington,Barbuda)

NWS National Hurricane Center Miami FL 500 PM AST Thu Sep 23, 2021 

Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory.

Sam’s estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual.

With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 11.1N  39.7W   50 KT  60 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 12H  24/0600Z 11.5N  41.9W   60 KT  70 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 24H  24/1800Z 11.9N  44.2W   75 KT  85 MPH - Category 1 (ESE Long Bay,Barbados)
 36H  25/0600Z 12.3N  46.0W   90 KT 105 MPH - Category 2 (ESE Grand'Rivière,Martinique)
 48H  25/1800Z 12.6N  47.4W  100 KT 115 MPH - Category 3 (ESE La Désirade,Guadeloupe)
 60H  26/0600Z 13.1N  48.7W  105 KT 120 MPH - Category 3 (ESE Long Bay,Barbados)
 72H  26/1800Z 13.7N  50.0W  110 KT 125 MPH - Category 3 (ESE La Désirade,Guadeloupe)
 96H  27/1800Z 15.2N  52.5W  110 KT 125 MPH - Category 3 (ESE Penville,Dominica)
120H  28/1800Z 17.5N  55.5W  110 KT 125 MPH - Category 3 (ESE Codrington,Barbuda)

NWS National Hurricane Center Miami FL 1100 AM AST Thu Sep 23, 2021

Overnight and this morning, the satellite structure of the tropical cyclone has been improving, with prominent curved bands, and obvious low-level cloud motions that indicate the circulation is becoming established within the convection. In particular, an SSMIS microwave pass at 0653 UTC indicated a substantial improvement in the convective structure, with a well-defined curved band wrapping three-quarters of the way around the center in both the 91- and 37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated a tight, well-defined circulation had formed, with peak winds of 44-kt on the north side of the vortex. Subjective Dvorak satellite intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from TAFB. In addition, the latest objective intensity estimates from UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given the recent scatterometer data, the intensity has been set to 45-kt for this advisory. Thus, Tropical Depression 18 has been upgraded to Tropical Storm Sam. It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion at 280/14 kt. A prominent mid-level ridge remains entrenched to the north of the cyclone, and this feature should continue to steer Sam to the west-northwest over the next 2-3 days. Over this period, the ridge orientation actually shifts more westward ahead of the cyclone as a deep-layer trough digs in well northeast of Sam. This gradual change in the synoptic pattern should also result in a slowdown in Sam’s forward motion over the next 72 hours. While the track guidance remains in good agreement over this period, larger spread begins to take shape beyond the day 3 forecast. Similar to yesterday, the ECMWF model and its ensemble mean flanks the southwest side of the guidance envelop, while the GFS is on the northeast side by day 5. These differences appear to be partially related to the mid-level ridge intensity on the western end, which could begin to be eroded by a mid-latitude trough off the eastern United States in 120-h. In addition, differences in both the size and intensity of Sam may also play a role in its ultimate track evolution. For this advisory, the latest NHC track forecast remains close to the consensus aids TCVN and TCVE, which represents a slight shift westward and a bit faster motion compared to the previous forecast. Because Sam is forecast to slow down, it remains too early to determine what impacts, direct or indirect, could be felt by the Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that the low-level center has become better aligned with the mid-level vortex, perhaps a bit earlier than anticipated given the scatterometer data from last night. This improvement in structure, combined with favorable low vertical wind shear under 10 kt and warm 28-29 C sea-surface temperatures, argues for significant intensification. In fact, the GFS-SHIPS rapid intensification index now give Sam a 39 percent chance of a 65-kt increase over the next 72 hours, which is more than 7 times the climatological value. Thus, the NHC intensity forecast has been raised quite a bit from the prior one, especially in the short-term. The latest forecast now makes Sam a hurricane in 36 hours, and a major hurricane in 72 hours. This intensity forecast is just a shade under the latest HCCA consensus aid. Thereafter, a more gradual intensification rate is forecast. By this period it is possible that Sam could undergo hard to predict inner-core changes such as eyewall replacement cycles. In addition, the ECMWF-SHIPS suggests that the shear may also increase a little in days 4-5 which also argues for a bit slower rate of intensification for this period.

FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 10.9N  38.1W   45 KT  50 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 12H  24/0000Z 11.3N  40.3W   50 KT  60 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 24H  24/1200Z 11.9N  42.9W   60 KT  70 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 36H  25/0000Z 12.3N  45.0W   70 KT  80 MPH - Category 1 (ESE Long Bay,Barbados)
 48H  25/1200Z 12.7N  46.7W   80 KT  90 MPH - Category 1 (ESE Long Bay,Barbados)
 60H  26/0000Z 13.2N  48.2W   90 KT 105 MPH - Category 2 (ESE Grand'Rivière,Martinique)
 72H  26/1200Z 13.7N  49.4W  100 KT 115 MPH - Category 3 (ESE La Désirade,Guadeloupe)
 96H  27/1200Z 15.3N  52.1W  105 KT 120 MPH - Category 3 (ESE Penville,Dominica)
120H  28/1200Z 17.4N  55.2W  110 KT 125 MPH - Category 3 (ESE Codrington,Barbuda)

NWS National Hurricane Center Miami FL 1100 PM AST Wed Sep 22, 2021

The depression’s organization has not improved since earlier this afternoon. An analysis of the ambiguities from 0000 UTC ASCAT-B data suggests that the surface circulation is just barely closed, with the center still attached to the northern end of a trough. Peak wind retrievals from the scatterometer pass were 25-30 kt, so the initial intensity remains 30 kt.

Based on the ASCAT data, the depression appears to be moving just north of due west, or 275/13 kt. Low- to mid-level ridging to the north is expected to maintain the cyclone on a westward heading for the next 36 hours, followed by a path toward the west-northwest from day 2 through day 5. The more notable part of the forecast is the forward speed. Global models are showing anomalously strong 500-mb ridging developing over the eastern and central Caribbean Sea in 2 to 3 days, which is likely to block the depression’s forward progress toward the end of the forecast period. In fact, the current NHC forecast has the system moving west-northwestward at only 7 or 8 kt well east of the Lesser Antilles on days 3 through 5. All this means that it will probably be a few more days before we have a better idea if and how the system might potentially threaten areas farther west. For the 5-day forecast period, there is a normal amount of spread among the track guidance, and the updated NHC forecast is very close to a blend of the TVCA and HCCA consensus aids. This solution is not too different from the previous forecast.

Overall, an environment of low shear, warm sea surface temperatures, and a moist, unstable atmosphere appears conducive for strengthening. However, there is some sort of signal being conveyed by the global models (e.g., the GFS, ECMWF, and UKMET) whereby the cyclone’s surface circulation does not really tighten up for another 24 to 48 hours. Given the system’s current structure, this scenario seems plausible. Therefore, the updated NHC intensity forecast shows a slower rate of strengthening during the first 48 hours, just a bit below the intensity consensus in deference to the global model solutions. After 48 hours, the official forecast converges on top of the previous NHC forecast and still brings the cyclone to hurricane, and then possibly major hurricane, strength by the end of the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0300Z 10.2N  35.0W   30 KT  35 MPH - Depression (WSW Mosteiros,Cape Verde)
 12H  23/1200Z 10.4N  37.0W   30 KT  35 MPH - Depression (WSW Mosteiros,Cape Verde)
 24H  24/0000Z 10.9N  39.6W   35 KT  40 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 36H  24/1200Z 11.4N  42.1W   45 KT  50 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 48H  25/0000Z 12.0N  44.3W   55 KT  65 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 60H  25/1200Z 12.6N  46.0W   70 KT  80 MPH - Category 1 (ESE Long Bay,Barbados)
 72H  26/0000Z 13.2N  47.5W   80 KT  90 MPH - Category 1 (ESE Long Bay,Barbados)
 96H  27/0000Z 14.5N  50.2W   90 KT 105 MPH - Category 2 (ESE Grand'Rivière,Martinique)
120H  28/0000Z 16.0N  53.3W  100 KT 115 MPH - Category 3 (ESE La Désirade,Guadeloupe)

NWS National Hurricane Center Miami FL 500 PM AST Wed Sep 22, 2021

The tropical wave that NHC has been monitoring over the last several days has gradually become better organized. The satellite structure in particular is quite impressive for a tropical depression, with both the 1800 UTC subjective Dvorak estimates from TAFB and SAB at T2.5/35 kt. This bigger question, however, was if the system possessed a well-defined closed earth-relative circulation. An ASCAT-C pass from earlier this morning hinted that the circulation was becoming better defined, with the development of westerly low-level winds to the south of the convective shield. This westerly low-level flow is also confirmed by atmospheric motion vectors (AMVs) available from the GOES-16 meso domain over the system. While the low-level circulation may still be somewhat broad, it now appears to be well-defined enough to mark the formation of a tropical cyclone. The initial intensity is set at 30-kt, in agreement with peak wind retrievals of 28-30 kt by the earlier scatterometer data.

The initial motion is estimated to be at 270/13 kt, though this is somewhat uncertain given that the center has only recently formed. An expansive mid-level ridge is located to the north and west of the cyclone, which should maintain its heading toward the west, though with a gradual gain in latitude as the system approaches the western extent of the ridge by day 5. The track guidance is in excellent agreement on this track evolution for the first three days, with just a bit more spread in the guidance thereafter. The ECMWF and its ensemble mean towards the end of the forecast is on the left side of the guidance envelope, while the GFS and HWRF models are currently on the right side. For the first NHC track forecast, I have elected to stay close to the track consensus aids, taking a blend of the HCCA and TCVN aids which are near the middle of the guidance envelope.

The environment ahead of the tropical depression appears quite favorable for intensification. Vertical wind shear is forecast by both the ECWMF- and GFS-based SHIPS guidance to stay at or under 10 kt for the next 3-5 days as the storm traverses warm 28-29 C sea surface temperatures. Most of the guidance responds to this environment by indicating strengthening, and the NHC intensity guidance follows suit, showing a steady increase in intensity throughout the forecast period. While it might take a bit of time for the formative low-level circulation to become vertically aligned with the mid-level center, once that occurs, it is possible a period of rapid intensification could occur during the five day forecast. The forecast intensity by 120 hours (100 kt) is on the higher end of the guidance envelope, but not as high as the latest HWRF or HAFS-B forecasts.

FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 10.1N  33.9W   30 KT  35 MPH - Depression (WSW Mosteiros,Cape Verde)
 12H  23/0600Z 10.1N  36.0W   35 KT  40 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 24H  23/1800Z 10.5N  38.6W   40 KT  45 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 36H  24/0600Z 11.0N  41.1W   50 KT  60 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 48H  24/1800Z 11.7N  43.4W   60 KT  70 MPH - Tropical Storm (WSW Mosteiros,Cape Verde)
 60H  25/0600Z 12.4N  45.3W   70 KT  80 MPH - Category 1 (ESE Long Bay,Barbados)
 72H  25/1800Z 13.0N  46.8W   80 KT  90 MPH - Category 1 (ESE Long Bay,Barbados)
 96H  26/1800Z 14.2N  49.5W   90 KT 105 MPH - Category 2 (ESE Grand'Rivière,Martinique)
120H  27/1800Z 16.0N  52.5W  100 KT 115 MPH - Category 3 (ESE La Désirade,Guadeloupe)

NWS National Hurricane Center Miami FL 800 AM EDT Wed Sep 22, 2021

Corrected location of Peter relative to Puerto Rico.

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Depression Peter, located a few hundred miles north of Puerto Rico, and on Tropical Depression Rose, located about 1000 miles west-northwest of the Cabo Verde Islands.

  • 1. Showers and thunderstorms continue to become better organized in association with a tropical wave located several hundred miles southwest of the Cabo Verde Islands. Environmental conditions are expected to remain conducive for additional development, and a tropical depression is expected to form within a day or two while the system moves westward at 10 to 15 mph across the eastern and central tropical Atlantic Ocean. Additional information on this system, including gale warnings, can be found in High Seas Forecasts issued by the National Weather Service. * Formation chance through 48 hours…high…90 percent. * Formation chance through 5 days…high...90 percent.
  • 2. A gale-force, non-tropical low pressure system, the remnants of Odette, is located a little less than 500 miles west-northwest of the westernmost Azores. This low could acquire some subtropical characteristics over marginally warm waters during the next few days while it makes a counter-clockwise loop over the north-central Atlantic Ocean. However, by the weekend, this system is expected to move into an environment of strong upper-level winds. Additional information on this system, including gale warnings, can be found in High Seas Forecasts issued by the National Weather Service. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium...50 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Tue Sep 21, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Depression Peter, located a couple of hundred miles northeast of Puerto Rico, and on Tropical Storm Rose, located about 1000 miles west-northwest of the Cabo Verde Islands.

  • 1. Showers and thunderstorms associated with a tropical wave located several hundred miles southwest of the Cabo Verde Islands have increased since this morning and are beginning to show signs of organization. Recent satellite wind data also indicate that a broad area of low pressure has formed, but the system lacks a well-defined center. Environmental conditions are expected to remain conducive for additional development, and a tropical depression is likely to form within the next day or two while the system moves westward at 10 to 15 mph across the eastern and central tropical Atlantic Ocean. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A gale-force, non-tropical low pressure system, the remnants of Odette, is located about 500 miles west of the westernmost Azores. This low could acquire some subtropical characteristics over marginally warm waters during the next few days while it executes a small counter-clockwise loop over the north-central Atlantic Ocean. However, by the end of the week, this system is expected to encounter more hostile environmental conditions. Additional information on this system, including gale warnings, can be found in High Seas Forecasts issued by the National Weather Service. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Tue Sep 21, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Storm Peter, located more than 100 miles northeast of Puerto Rico, and on Tropical Storm Rose, located over the eastern tropical Atlantic Ocean.

  • 1. A small but concentrated area of showers and thunderstorms is redeveloping along a tropical wave located a few hundred miles southwest of the Cabo Verde Islands. Environmental conditions are expected to become more conducive for development, and a tropical depression is likely to form by Thursday or Friday while the system moves westward at 10 to 15 mph across the eastern and central tropical Atlantic Ocean. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A gale-force, non-tropical low pressure system, the remnants of Odette, is located about 700 miles west-northwest of the westernmost Azores. This low could acquire some subtropical characteristics over marginally warm waters during the next few days while it executes a small cyclonic loop over the north-central Atlantic Ocean. However, by the end of the week, this system is expected to encounter more hostile environmental conditions. Additional information on this system, including gale warnings, can be found in High Seas Forecasts issued by the National Weather Service. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…medium...40 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Tue Sep 21 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Storm Peter, located about 100 miles north-northwest of the northern Leeward Islands, and on Tropical Storm Rose, located over the eastern tropical Atlantic Ocean.

  • 1. Showers and thunderstorms are currently somewhat limited associated with a tropical wave located a few hundred miles southwest of the Cabo Verde Islands. However, environmental conditions are expected to become more conducive for development, and a tropical depression is likely to form by Thursday or Friday while the system moves westward at 10 to 15 mph across the eastern and central tropical Atlantic Ocean. * Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A storm-force, non-tropical low pressure system, the remnants of Odette, is located about 700 miles west-northwest of the westernmost Azores. This low could acquire some subtropical characteristics over marginally warm waters during the next few days while it executes a small cyclonic loop over the north-central Atlantic Ocean. However, by the end of the week, this system is expected to encounter more hostile environmental conditions. Additional information on this system, including storm warnings, can be found in High Seas Forecasts issued by the National Weather Service. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…medium…40 percent.

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Video: Tracking the tropics: Sam falls apart as another system emerges near the Bahamas