Tropical Storm Helene

Tropical Storm Helene 1100 Hours September 16 2018
Tropical Storm Helene 1100 Hours September 16 2018

NWS National Hurricane Center Miami FL 1100 AM AST Sun Sep 16 2018

Helene’s (see 9 videos below) cloud pattern has transformed into that of an extratropical cyclone, with a large area of cold cloud tops located well to the north and northwest of the center. An ASCAT pass that caught the eastern portion of the circulation revealed maximum winds of 40 to 45 kt, therefore the initial wind speed remains 45 kt for this advisory. The global models suggest that the system will strengthen slightly today due to baroclinic processes, and this is reflected in the official forecast. Weakening is anticipated by late Monday, and the system is forecast to be absorbed by a larger low pressure to the northwest of the United Kingdom by Tuesday morning.

The cyclone is moving quickly northeastward or 055/25 kt. Now that the system is embedded within the mid-latitude westerlies, a northeastward motion with some additional increase in forward speed is expected over the next day or so. The global models remain in good agreement on the track of the post-tropical low, and the new official forecast is an update of the previous advisory.

This is the last NHC advisory on this system.

Storm Helene is expected to move northeastwards across the British Isles later Monday and early Tuesday before clearing into the northern North Sea by early Tuesday morning. A spell of strong winds is expected, initially mainly in the far southwest of England and across western Wales. The strongest winds then transfer northwards to be over northern England and, perhaps, the far south of Scotland, during the early hours of Tuesday. Winds are likely to gust into the 40s or low 50s mph quite widely across the warning area. Meanwhile, in some Irish Sea coastal areas, most likely in Wales and northwest England, gusts to 55-65 mph are likely with possible isolated gusts in excess of 70 mph in the most exposed places. High gusts in the 50s or low 60s mph area also possible over high ground in northern England during the early hours of Tuesday.

NWS National Hurricane Center Miami FL 500 PM AST Sat Sep 15 2018

The cloud pattern of Tropical Storm Helene is gradually decaying and transitioning to a shear pattern, with the low-level center occasionally visible through breaks in the cirrus overcast. The initial intensity is reduced to 55 kt in best agreement with the CIMSS satellite consensus. Helene is moving toward colder water and into increasing shear associated with the mid-latitude westerlies. This should cause continued weakening and, as Helene merges with a frontal system, extratropical transition in about 36 h. The forecast will continue to call for Helene or its remnants to be absorbed into a large non-tropical low over the North Atlantic between 72-96 h, and the latest global model guidance suggests this could happen earlier than forecast.

The initial motion is now 050/19. A mostly northeastward motion at a faster forward speed should occur during the remainder of the cyclone’s life as it becomes more embedded in the westerlies. On the forecast track, Helene should pass near or over the western Azores during the next several hours, and head toward Ireland and the United Kingdom Sunday night and Monday. There are again no significant changes to either the track guidance or the official forecast track since the last advisory.

Tropical storm conditions are spreading over portions of the Azores and should continue through tonight.

Helene is also expected to be a storm-force post-tropical cyclone when it approaches Ireland and the United Kingdom in two to three days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene.

FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 40.2N  32.1W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  16/0600Z 41.9N  29.2W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  16/1800Z 44.4N  24.1W   55 KT  65 MPH - Tropical Storm (WSW A Coruña, Spain)
 36H  17/0600Z 47.2N  17.9W   55 KT  65 MPH - Tropical Storm (WNW A Coruña, Spain)
 48H  17/1800Z 51.2N  11.1W   50 KT  60 MPH - Tropical Storm (WSW Baltimore, Ireland)
 72H  18/1800Z 61.0N   1.0E   50 KT  60 MPH - Tropical Storm (WNW Bergen, Norway)
 96H  19/1800Z...ABSORBED BY EXTRATROPICAL LOW

NWS National Hurricane Center Miami FL 1100 AM AST Sat Sep 15 2018

The cloud pattern of Helene is starting to show signs of decay, with the central convection decreasing since the previous advisory and the low-level center becoming partly exposed in a region between convective bands. The initial intensity is held at 60 kt for this advisory in best agreement with the CIMSS satellite consensus, but it is possible this is generous. The 34-kt wind radii in the southeastern quadrant were revised based on a partial ASCAT pass.

Helene is moving toward colder water and into an area of strong shear associated with the mid-latitude westerlies. This should cause the system to weaken and, as it merges with a frontal system, become an extratropical cyclone in about 36 h. The global models forecast Helene to be absorbed into a large non-tropical low over the far north Atlantic after it passes Ireland and the United Kingdom, and thus the system is forecast to dissipate completely between 72-96 h.

Helene has turned northeastward with the initial motion now 035/18. A faster motion toward the northeast should occur during the remainder of the cyclone’s life as it becomes more embedded in the westerlies. On the forecast track, Helene should pass near or over the western Azores later today or tonight, and head toward Ireland and the United Kingdom late this weekend and early next week. There are no significant changes to either the track guidance or the official forecast track since the last advisory.

Tropical storm conditions should spread over portions of the Azores during the next several hours and continue through tonight.

Helene is also expected to be a storm-force post-tropical cyclone when it approaches Ireland and the United Kingdom in two to three days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene.

FORECAST POSITIONS AND MAX WINDS

INIT  15/1500Z 38.8N  34.0W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  16/0000Z 40.8N  31.4W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  16/1200Z 43.0N  27.2W   55 KT  65 MPH - Tropical Storm (ENE Angra do Heroísmo, Azores)
 36H  17/0000Z 45.5N  21.6W   55 KT  65 MPH - Tropical Storm (WSW A Coruña, Spain)
 48H  17/1200Z 49.0N  15.0W   55 KT  65 MPH - Tropical Storm (WSW Baltimore, Ireland)
 72H  18/1200Z 59.0N   1.5W   50 KT  60 MPH - Tropical Storm (ESE Everbay, Scotland)
 96H  19/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 AM AST Sat Sep 15 2018

Helene appears to be maintaining its strength. Recent microwave images show that the storm has an eye feature, but the vortex is significantly tilted in the vertical due to southwesterly shear. Deep convection is most organized in bands to the north and west of the center. Based on the tropical storm’s appearance in microwave images, the initial intensity is again held at 60 kt, near the high end of the satellite intensity estimates. Helene is expected to gradually weaken during the next few days due to the influences of southwesterly shear, drier air, and much cooler SSTs along the forecast track. These conditions should also cause Helene to lose tropical characteristics, and the storm is forecast become extratropical in 24 to 36 hours.

The tropical storm is moving north-northeastward at a fairly fast pace, 19 kt. The center of Helene is expected to pass to the west of the Azores by tonight. However, Helene has a fairly large wind field, and tropical-storm-force winds are expected across the western Azores as the storm passes by. A shortwave trough to the northwest of the system is expected to approach Helene by tonight, and that should cause the cyclone to turn northeastward and then speed up in that direction as it heads toward Ireland and the United Kingdom late this weekend and early next week. The models are in fair agreement, and only small changes were made to the previous NHC track forecast.

Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in two to three days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 37.7N  34.8W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  15/1800Z 39.7N  32.7W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  16/0600Z 41.8N  29.2W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  16/1800Z 44.1N  24.3W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  17/0600Z 46.7N  18.1W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  18/0600Z 55.8N   5.0W   50 KT  60 MPH - Tropical Storm (ESE Kerrycroy, Scotland)

NWS National Hurricane Center Miami FL 1100 PM AST Fri Sep 14 2018

Tropical Storm Helene  – Data from a timely ASCAT pass indicate that Helene still carries winds of 60 kt, primarily in the eastern semicircle. The cloud pattern continues to show a low-level center just south of the main convection, marking the strong shear which is affecting the system. Guidance indicates that Helene will not change much in intensity in the next 12 hours, and only a slight decrease in the winds is anticipated after that time. The wind field is forecast to expand mainly in the southeast quadrant, which is the area of the cyclone that most likely will affect the Azores. After passing to the north of, or near, the Azores in about 24 hours, Helene will be moving over cold waters and become post-tropical. The cyclone should be absorbed by a larger extratropical low, as the cyclone quickly approaches Ireland and the United Kingdom.

Satellite fixes indicate that Helene is moving toward the north-northeast or 030 degrees at about 16 kt. Helene is embedded within the fast southwesterly flow ahead of a sharp mid-level trough and this pattern should steer the cyclone toward the northeast with a gradual increase in forward speed. Track guidance is very consistent with this solution, and the NHC forecast continues in the middle of the tight guidance envelope.

Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0300Z 35.8N  35.8W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  15/1200Z 38.0N  34.5W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  16/0000Z 40.3N  31.2W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  16/1200Z 42.5N  26.5W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  17/0000Z 45.0N  21.5W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  18/0000Z 51.5N   9.0W   50 KT  60 MPH - Tropical Storm (S Owenahincha, Ireland)
 96H  19/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM AST Fri Sep 14 2018

The 40-45 kt of southwesterly vertical shear has really taken its toll on Helene’s  cloud pattern. A severe tilt toward the northeast is evident in conventional and microwave imagery. Despite the sheared structure of the cyclone, the initial intensity is held at 60 kt due to the earlier scatterometer pass showing that value in the northeast quadrant, and a 1603 UTC SATCON analysis that yields an intensity estimate of 58 kt.

Little change in strength is forecast during the next 12 hours as Helene remains over 26 to 27C sea surface temperatures. Beyond that brief period, the cyclone should weaken a bit as it completes extratropical transition while passing just to the northwest of the Azores Islands and traversing much cooler waters. The European models and the Canadian still show Helene being absorbed by a larger baroclinic system in 4 days, as the cyclone quickly approaches Ireland and the United Kingdom. The GFS, which earlier indicated a different solution resulting in Helene taking a more north- northeastward to northwest track around the periphery of a larger, complex non-tropical low, now shows Helene merging with a front, similar to the European and Canadian scenario.

A series of recent microwave passes reveal that Helene’s surface circulation has been tracking quite a bit to the left of the previous forecasts. Consequently, an adjustment to the best track was made to conform more with the microwave overpasses. The initial motion is estimated to be northward, or 010/16 kt. It is also quite possible that there maybe some binary involvement between Helene and Tropical Storm Joyce that could be influencing a motion just to the left of track. In any event, a north-northeast to northeast turn is expected over the weekend as the cyclone accelerates within the mid-latitude westerlies. The NHC forecast is again adjusted to the left through 36 hours, due to the much welcomed microwave images, and is in best agreement with TVCN multi-model consensus.

Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene.

FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 34.1N  36.8W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  15/0600Z 36.4N  36.0W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  15/1800Z 38.9N  33.7W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  16/0600Z 41.0N  30.0W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  16/1800Z 43.3N  25.2W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  17/1800Z 49.5N  12.9W   50 KT  60 MPH - Tropical Storm (WSW Baltimore, Ireland)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Sep 14 2018

Conventional satellite imagery and a 1147 UTC METOP-B AMSU microwave image show a deep convective curved band wrapping around the surface circulation in the west semicircle. Additionally, a recent 1148 UTC ASCAT-B scatterometer pass indicates that Helene is a little stronger than earlier estimated. The data showed a number of 60 kt winds in the northeast quadrant. Based on these data, Helene is still classified as a tropical cyclone, and the initial intensity is bumped up to 60 kt for this advisory.

The large-scale models and the statistical intensity guidance indicate little change in strength during the next 36 hours or so. After that time, some gradual weakening is forecast as Helene completes extratropical transition while passing just to the north of the Azores Islands. The ECMWF, UKMET, and the CMC all show Helene absorbed by a larger baroclinic system just beyond the 72 hour period, as the cyclone moves over Ireland and the United Kingdom.

The initial motion is estimated to be northward, or 010/20 kt, within the deep-layer southerly flow produced by a mid-tropospheric trough to the west. A north-northeast to northeast turn is expected over the weekend as the cyclone quickly moves within the mid-latitude westerlies. Only a slight adjustment to the left of the previous track forecast was made through 48 hours. The NHC official forecast is based primarily on the TVCN multi-model consensus. An orange wind warning is in effect for the Azores, equivalent to a tropical storm warning. Tropical Storm conditions with gusty winds and heavy rains are likely over the weekend as Helene passes the islands, regardless of whether the cyclone is a tropical storm or post-tropical.

Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/.

Tropical storm Helene Arrival Times 0500 Hours September 14 2018
Tropical storm Helene Arrival Times 0500 Hours September 14 2018
FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 32.9N  36.3W   60 KT  70 MPH - Tropical Storm (WSW Brest, France)
 12H  15/0000Z 35.5N  35.5W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  15/1200Z 38.4N  33.4W   55 KT  65 MPH - Tropical Storm (WSW Fajã Grande, Azores)
 36H  16/0000Z 40.7N  30.0W   50 KT  60 MPH - Tropical Storm (ENE Vila do Corvo, Azores)
 48H  16/1200Z 42.7N  25.7W   50 KT  60 MPH - Tropical Storm (ESE Vila do Corvo, Azores)
 72H  17/1200Z 47.4N  16.1W   50 KT  60 MPH - Tropical Storm (WSW Brest France)
 96H  18/1200Z...ABSORBED BY FRONT

NWS National Hurricane Center Miami FL 500 AM AST Fri Sep 14 2018

Helene is looking increasingly like an extratropical cyclone, with a rain shield extending from its northwest quadrant and a line of clouds to the southeast, probably associated with a convergence zone, that looks like a developing a front. However, earlier ASCAT data suggested that the cyclone still had a compact wind field with maximum winds of around 55 kt on the west side, and the most recent available AMSU data still showed a distinct warm core.

The extratropical transition (ET) process will likely be completed within the next 48 h, around the time that Helene will likely pass near or over the Azores. While this process occurs, little weakening is forecast by the various dynamical models, but gradual weakening should occur once ET is completed. By the end of the forecast period, nearly all of the global models depict the cyclone being absorbed into a larger mid-latitude low pressure system over the far northeast Atlantic.

Helene continues to accelerate northward, and the initial motion is 010/20 kt. Almost no change has been made to the track forecast. Helene is still forecast to move quickly northward to north-northeastward between a mid-level ridge to its east and a sharp mid-latitude trough to its west. A northeastward turn should occur over the weekend as Helene reaches the northern extent of the ridge and becomes embedded within the mid-latitude westerlies. The official track forecast remains near the various consensus aids.

The government of Portugal has issued an orange wind warning for the Azores, equivalent to a tropical storm warning. Tropical Storm conditions with gusty winds and heavy rains are likely over the weekend as Helene passes the islands, regardless of whether the cyclone is a tropical storm or post-tropical.

Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 30.6N  36.0W   55 KT  65 MPH - Tropical Storm (WSW Ponta Delgada, Azores)
 12H  14/1800Z 33.4N  35.3W   55 KT  65 MPH - Tropical Storm (WSW Ponta Delgada, Azores)
 24H  15/0600Z 36.7N  33.8W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz Das Flores, Azores)
 36H  15/1800Z 39.3N  30.8W   55 KT  65 MPH - Tropical Storm (ESE Santa Cruz Das Flores, Azores)
 48H  16/0600Z 41.3N  26.9W   50 KT  60 MPH - Tropical Storm (ENE Angra do Heroísmo, Azores)
 72H  17/0600Z 45.5N  17.7W   50 KT  60 MPH - Tropical Storm (WSW Brest, France)
 96H  18/0600Z 51.5N   8.0W   45 KT  50 MPH - Tropical Storm (ESE Rennies. Ireland)

NWS National Hurricane Center Miami FL 1100 PM AST Thu Sep 13 2018

Helene’s cloud pattern is beginning to have the appearance of a cyclone in extratropical transition with the rain shield limited to the northwest quadrant. The Dvorak numbers have decreased, but very recent scatterometer data show winds of around 55 kt associated with Helene’s circulation, and this is the intensity value used in this advisory. Most of the guidance suggests that Helene’s strength should remain steady through the forecast period, perhaps due to baroclinic forcing. On this basis, the NHC forecast keeps Helene with the same intensity through the forecast period, but as a post-tropical cyclone beyond 3 days.

Helene has increased its forward speed and is now moving toward the north or 010 degrees at about 18 kt. The flow ahead of a sharp mid-level trough is well established, and this pattern will continue to steer Helene toward the north and then northeast during the next few days. This is consistent with most of the track guidance which is indeed tightly clustered. The NHC forecast is not very different from the previous one, and is in between the corrected consensus HCCA and the multi-model consensus aids.

 

 

 

 

 

 

 

 

 

 

 

Helene is moving northward at about 12 kt between the subtropical ridge to its east and a deep-layer trough to its west. The system should accelerate and turn toward the northeast over the next few days as it rounds the subtropical ridge and starts getting picked up by the mid-latitude westerlies. The official track forecast is based upon the tightly clustered variable consensus technique (TVCN) and is nearly the same as the previous forecast.

FORECAST POSITIONS AND MAX WINDS
INIT  14/0300Z 28.6N  36.5W   55 KT  65 MPH - Tropical Storms (WNE Santa Cruz de Tenerife, Canary Islands)
 12H  14/1200Z 31.4N  35.9W   55 KT  65 MPH - Tropical Storms (WNE Santa Cruz de Tenerife, Canary Islands)
 24H  15/0000Z 34.8N  34.7W   50 KT  60 MPH - Tropical Storms (WNW Santa Cruz das Flores, Azores)
 36H  15/1200Z 37.8N  32.8W   50 KT  60 MPH - Tropical Storms (WNW Santa Cruz das Flores, Azores)
 48H  16/0000Z 40.0N  29.0W   50 KT  60 MPH - Tropical Storms (ENE Ponta Delgada, Azore)
 72H  17/0000Z 43.0N  21.5W   50 KT  60 MPH - Tropical Storms (W Pontevedra, Spain)
 96H  18/0000Z 46.5N  15.0W   50 KT  60 MPH - Tropical Storms (NW La Rochelle, France)
120H  19/0000Z 53.0N   5.0W   50 KT  60 MPH - Tropical Storms (W Holyhead, Wales)

NWS National Hurricane Center Miami FL 1100 AM AST Thu Sep 13 2018

Tropical Storm Helene continues with very limited deep convection, only observed this morning in the northwestern quadrant. Dvorak estimates from TAFB and SAB are dropping, and a blend of these with the CIMSS SATCON indicates an intensity of 60 kt. Thus Helene has weakened to a tropical storm.

Strong tropospheric vertical shear along with only lukewarm waters are likely contributing toward Helene’s weakening. The shear should further increase during the next two days, though the sea surface temperatures will rebound up to 27C along with an increase in low-level moisture. A slow weakening is thus expected. Beginning in about three days, Helene will commence baroclinic transition, and it is expected to become an extratropical cyclone by 96 hours. This forcing should preclude any additional weakening through the remainder of the forecast period. The official intensity forecast is similar to the previous advisory. The prediction is based on a blend of the stronger HWRF dynamical model and the weaker SHIPS/LGEM statistical schemes for the next couple of days, and is based on a blend of the GFS/ECMWF/UKMET global models at the extended lead times.

Helene is moving northward at about 12 kt between the subtropical ridge to its east and a deep-layer trough to its west. The system should accelerate and turn toward the northeast over the next few days as it rounds the subtropical ridge and starts getting picked up by the mid-latitude westerlies. The official track forecast is based upon the variable consensus technique (TVCN) and is just north of the previous forecast through three days. The guidance suite remains tightly clustered and indicates that Helene will pass near the Azores in 2 or 3 days. Interests in those islands should closely monitor the progress of Helene.

FORECAST POSITIONS AND MAX WINDS

INIT  13/1500Z 24.8N  37.3W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz de Tenerife, Canary Islands)
 12H  14/0000Z 27.2N  37.1W   55 KT  65 MPH - Tropical Storm (SW Las Palmas, Gran Canaria)
 24H  14/1200Z 30.8N  36.6W   50 KT  60 MPH - Tropical Storm (NW Las Palmas, Gran Canaria)
 36H  15/0000Z 34.2N  35.5W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  15/1200Z 37.2N  33.3W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  16/1200Z 40.5N  26.0W   50 KT  60 MPH - Tropical Storm (NW Angra do Heroísmo, Azores)
 96H  17/1200Z 44.0N  18.0W   50 KT  60 MPH - Tropical Storm (WNW A Coruña, Spain)
120H  18/1200Z 51.5N   9.0W   50 KT  60 MPH - Tropical Storm (WSW Long Strand, Ireland)

NWS National Hurricane Center Miami FL 500 AM AST Thu Sep 13 2018

Helene’s surface circulation has become fully exposed this morning with the center decoupled about 110 miles south of edge of the remaining deep convection. Subjective and objective satellite intensity estimates have decreased and support lowering the initial intensity to 65 kt.

Statistical and global model guidance show the southwesterly shear increasing significantly in 12 hours or so, however, the 26 to 27C sea surface temperatures should govern the weakening trend a little. In 3 days, Helene is forecast to move over much cooler waters while the cyclone becomes embedded in the hostile mid-latitude upper-level westerlies. Subsequently, further weakening is expected at a faster pace. By day 4, the large-scale models all show Helene acquiring extratropical cyclone characteristics as it moves toward Ireland and the United Kingdom. The NHC intensity forecast is similar to the previous advisory, and is based primarily on the NOAA-HCCA and the IVCN consensus models.Tropical Storm Helene Tropical Force Winds 0200 Hours September 13 2018

Helene is still moving northward, or 355/12 kt, within the deep-layer southerly flow produced by an amplifying mid-Atlantic trough to the west of the cyclone. In 48 hours, Helene is forecast to turn north-northeastward followed by a turn toward the northeast with an increase in forward motion at the 72 hour period. The guidance suite remains tightly clustered through day 4, with some slight spread at day 5, and indicates that Helene will pass near the Azores in about 3 days. Interests in those islands should closely monitor the progress of Helene over the next few days. The official forecast is basically an update of the previous one, and lies close to the TVCN multi-model consensus through the entire forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 23.5N  37.3W   65 KT  75 MPH - Category 1 (WSW Santa Cruz de Tenerife, Canary Islands)
 12H  13/1800Z 25.7N  37.4W   60 KT  70 MPH - Tropical Storm (WSW Santa Cruz de Tenerife, Canary Islands)
 24H  14/0600Z 29.1N  36.9W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz de Tenerife, Canary Islands)
 36H  14/1800Z 32.5N  36.2W   55 KT  65 MPH - Tropical Storm (WSW Ponta Delgada, Azores)
 48H  15/0600Z 35.8N  34.7W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores )
 72H  16/0600Z 40.7N  28.2W   50 KT  60 MPH - Tropical Storm (N Santa Cruz da Graciosa, Azores )
 96H  17/0600Z 44.6N  19.5W   50 KT  60 MPH - Tropical Storm (WNW A Coruña, Spain)
120H  18/0600Z 51.1N  10.8W   45 KT  50 MPH - Tropical Storm (WSW Castletownbere, Ireland)

NWS National Hurricane Center Miami FL 500 AM AST Wed Sep 12 2018

Earlier this morning, Helene  looked quite ragged in IR imagery. The eye became poorly defined and cloud tops were steadily warming. However, beginning around 0500 UTC, a new burst of convection began wrapping around the previously open western portion of Helene’s inner core. Objective and subjective Dvorak current intensity estimates range from 77 to 91 kt, and the initial intensity is therefore set at 80 kt as a blend of all of these data.

Despite the fact that Helene appears to be on the upswing at the moment, the cyclone is still passing over sub-26 deg C waters and all of the intensity guidance shows weakening for the next 24 h. Beyond that time, there is still some spread among the intensity models, though it has decreased since the last advisory. Helene will begin interacting with a mid-latitude trough over the north-central Atlantic, which will result in an increase in shear over the hurricane. However, Helene will also begin to move over warmer waters and could get some baroclinic support from the upper-level trough. Hurricane Helene Tropical Force Winds 0200 Hours September 12 2018Overall the intensity guidance is a little lower than before, and now the NHC intensity forecast is near the middle of the intensity guidance envelope.

Once again, no meaningful change has been made to the NHC track forecast which continues to closely follow the HFIP Corrected Consensus. Helene is moving north-northwestward, and should gradually turn northward, and eventually northeastward, as the cyclone is steered between a mid-level ridge to the east and the aforementioned mid-latitude trough to northwest. The global models remain in fairly good agreement on the track of Helene, especially through 72-96 h, when Helene is forecast to pass near the Azores, and interests in those islands should closely monitor the progress of Helene over the next several days.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0900Z 19.2N  35.7W   80 KT  90 MPH - Category 1 (WNW Mindelo, Cape Verde)
 12H  12/1800Z 20.7N  36.6W   75 KT  85 MPH - Category 1 (WNW Mindelo, Cape Verde)
 24H  13/0600Z 22.9N  37.2W   65 KT  75 MPH - Category 1 (WNW Mindelo, Cape Verde)
 36H  13/1800Z 25.5N  37.3W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz de Tenerife, Canary Islands)
 48H  14/0600Z 28.9N  36.9W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz de Tenerife, Canary Islands)
 72H  15/0600Z 36.0N  34.3W   50 KT  60 MPH - Tropical Storm (ESE Santa Cruz das Flores, Azores)
 96H  16/0600Z 41.0N  28.5W   50 KT  60 MPH - Tropical Storm (ENE Vila do Corvo, Azores)
120H  17/0600Z 45.5N  19.0W   50 KT  60 MPH - Tropical Storm (WSW Bordeaux, France)

NWS National Hurricane Center Miami FL 500 PM AST Tue Sep 11 2018

Hurricane Helene   is feeling the effects of cooler waters, and the convection surrounding the distinct eye is weaker that earlier today. Dvorak numbers are decreasing, and only support an initial intensity of 90 kt. Both cooler waters and high shear will continue to impact the hurricane, and Helene will probably weaken to a tropical storm in about 36 hours or so. The NHC forecast calls for a decrease in the winds at the same rate as the intensity consensus aids.

Helene is already on the southwestern side of the subtropical ridge, and it is now moving toward the northwest or 320 degrees at 10 kt. Global models continue to develop a large mid-level trough over eastern Atlantic, and the flow associated with this trough should steer Helene north-northwestward and northward during the next day to so. The cyclone should then recurve northeastward with an increase in forward speed and by Friday, a weakened Helene should be approaching the Azores. At the very long range, the cyclone will begin to lose tropical characteristics over the even colder waters of the north Atlantic. There has been little change in the track forecast since models have been very consistent with the current solution.

FORECAST POSITIONS AND MAX WINDS

INIT  11/2100Z 17.2N  34.9W   90 KT 105 MPH - Category 2 (WNW Mindelo, Cape Verde)
 12H  12/0600Z 18.3N  36.0W   80 KT  90 MPH - Category 1 (WNW Mindelo, Cape Verde)
 24H  12/1800Z 20.2N  37.2W   70 KT  80 MPH - Category 1 (WNW Mindelo, Cape Verde)
 36H  13/0600Z 22.3N  37.8W   60 KT  70 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 48H  13/1800Z 25.0N  38.0W   50 KT  60 MPH - Tropical Storm (WSW Villa de Valverde, Spain )
 72H  14/1800Z 31.5N  37.0W   45 KT  50 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 96H  15/1800Z 39.0N  32.5W   45 KT  50 MPH - Tropical Storm (WNW Fajã Grande, Azores)
120H  16/1800Z 43.5N  25.0W   45 KT  50 MPH - Tropical Storm (WNW A Coruña, Spain)

NWS National Hurricane Center Miami FL 1100 AM AST Tue Sep 11 2018

It appears that Helene already peaked in intensity earlier this morning with 95 kt. The cloud pattern has not changed much, and still consists of a large eye surrounded by deep convection. However, these convective tops have warmed a little, and consequently Dvorak numbers have remained steady. From now on, Helene will begin to move over increasingly cooler waters, and become embedded within higher shear in about a day. These environmental conditions should result in weakening, and the NHC forecast calls for a decrease in the winds at the same rate as the intensity consensus aids.

Helene has turned toward the northwest or 310 degrees at 10 kt. Models continue to erode the ridge to the north of the cyclone, and are also developing a large mid-level trough over eastern Atlantic. This forecast flow pattern will force Helene to turn more toward the north-northwest and north ahead of the trough for the next 2 to 3 days. After that time, the cyclone will recurve in the mid- latitude westerlies, and will begin to lose tropical characteristics at the very long ranges. Track models are in very good agreement with this solution. The NHC forecast is in the middle of the guidance envelope and in between HCCA and the multi-model consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  11/1500Z 16.5N  34.3W   95 KT 110 MPH - Category 2 (WSW Mindelo, Cape Verde)
 12H  12/0000Z 17.5N  35.5W   90 KT 105 MPH - Category 2 (WNW São Filipe, Cape Verde)
 24H  12/1200Z 18.9N  36.9W   80 KT  90 MPH - Category 1 (WNW Mindelo, Cape Verde)
 36H  13/0000Z 20.7N  37.8W   70 KT  80 MPH - Category 1 (WNW Mindelo, Cape Verde)
 48H  13/1200Z 22.5N  38.2W   60 KT  70 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 72H  14/1200Z 27.5N  38.0W   50 KT  60 MPH - Tropical Storm (WSW Los Llanos, Spain)
 96H  15/1200Z 35.0N  35.0W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
120H  16/1200Z 40.0N  30.0W   50 KT  60 MPH - Tropical Storm (ESE Vila do Corvo, Portugal)

NWS National Hurricane Center Miami FL 1100 AM AST Mon Sep 10 2018

Helene’s cloud pattern has become much better organized during the past few hours, with a clear 20 n mi wide eye seen in Meteosat infrared imagery, and a clear eye noted in a 1135 UTC AMSU overpass. A Dvorak classifications from TAFB and SAB were both T5.0/90 knots at 1200 UTC, and that is the initial intensity for this advisory. Helene will remain in an environment that should support some additional strengthening for the next 12 hours or so, with very low shear and SSTs of 27C or above. After that time, the SSTs cool quickly and the shear increases dramatically by 36 hours, which should induce steady weakening. Late in the period, the shear continues, and while SSTs increase the atmosphere dries out, and the intensity is held steady at day 5. The new NHC intensity forecast is above most of the guidance through 24 hours to account for the recent intensification of the storm, but then is close to or slightly above HCCA and IVCN.

The initial motion estimate is 285/14. The subtropical ridge north of Helene will gradually weaken as an upper-level trough digs southward around 40W. This pattern will cause Helene to slow down and gradually recurve during the forecast period, with a faster northward motion expected at days 4 and 5 as Helene interacts with the aforementioned upper-level trough. While there is significant variability between the various global models in the exact evolution of the upper-level pattern, the track model guidance is in generally good agreement on this evolution, although the UKMET, its ensemble mean and the GEFS mean are notable outliers to the right. The new NHC track was not changed much from the previous one and lies near the middle of the guidance envelope and is close to HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.6N  30.0W   90 KT 105 MPH - Category 2 (ESE São Filipe, Cape Verde)
 12H  11/0000Z 15.2N  32.0W  100 KT 115 MPH - Category 3 (ENE São Filipe, Cape Verde)
 24H  11/1200Z 15.9N  34.3W  100 KT 115 MPH - Category 3 (ESE Mindelo, Cape Verde)
 36H  12/0000Z 16.8N  36.1W   90 KT 105 MPH - Category 2 (ESE Mindelo, Cape Verde)
 48H  12/1200Z 18.0N  37.7W   75 KT  85 MPH - Category 1 (ENE Mindelo, Cape Verde)
 72H  13/1200Z 21.5N  39.5W   60 KT  70 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 96H  14/1200Z 27.0N  40.5W   50 KT  60 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
120H  15/1200Z 32.5N  40.5W   50 KT  60 MPH - Tropical Storm (ESE Funchal, Portugal)

NWS National Hurricane Center Miami FL 500 PM AST Sun Sep 09 2018

DISCUSSION AND OUTLOOK

At 500 PM AST (2100 UTC), the center of Hurricane Helene was located near latitude 13.4 North, longitude 26.0 West. Helene is moving toward the west near 13 mph (20 km/h), and a turn toward the west-northwest is expected on Monday, with a continued west-northwestward motion through Tuesday. On the forecast track, the center of Helene will continue to move away from the Cabo Verde Islands.

Maximum sustained winds have increased to near 75 mph (120 km/h) with higher gusts. Additional strengthening is forecast during the next 48 hours.

Hurricane-force winds extend outward up to 15 miles (30 km) from the center and tropical-storm-force winds extend outward up to 80 miles (130 km). The estimated minimum central pressure is 992 mb (29.30 inches).

HAZARDS AFFECTING LAND

RAINFALL: Helene is expected to produce total rain accumulations of 2 to 4 inches over the southern Cabo Verde Islands through today, with isolated maximum amounts of 6 inches. This rainfall may produce life-threatening flash floods.

NWS National Hurricane Center Miami FL 200 PM AST Sun Sep 09 2018

DISCUSSION AND OUTLOOK

At 200 PM AST (1800 UTC), the center of Tropical Storm Helene was located near latitude 13.3 North, longitude 25.5 West. Helene is moving toward the west near 13 mph (20 km/h), and this motion is expected to continue through today. A turn toward the west-northwest is forecast to occur by Monday.

Maximum sustained winds are near 65 mph (100 km/h) with higher gusts. Strengthening is forecast during the next 48 hours, and Helene is expected to become a hurricane later today.

Tropical-storm-force winds extend outward up to 80 miles (130 km) from the center. The estimated minimum central pressure is 997 mb (29.44 inches).

HAZARDS AFFECTING LAND

WIND: Tropical storm conditions are expected within portions of the warning area today. Hurricane conditions are also possible in the southernmost Cabo Verde Islands today.

RAINFALL: Helene is expected to produce total rain accumulations of 2 to 4 inches over the southern Cabo Verde Islands through today, with isolated maximum amounts of 6 inches. This rainfall may produce life-threatening flash floods.

NWS National Hurricane Center Miami FL 1100 AM AST Sun Sep 09 2018

Helene  continues to exhibit a fairly well-organized cloud pattern, but does not yet have a well-defined inner core. The current intensity estimate, 55 kt, is based on a blend of subjective and objective Dvorak estimates and is also close to the latest SATCON value. The tropical cyclone is expected to remain over warm waters with low to moderate vertical wind shear for the next couple of days. Therefore, strengthening is likely through the early part of this week. Later in the period, marginal SSTs and increasing south-southwesterly shear should induce weakening. The official wind speed forecast is on the high side of the numerical intensity guidance.

Helene continues to move westward, or about 270/11 kt. The system should move westward to west-northwestward to the south of a mid-level ridge over the eastern Atlantic over the next 48 hours or so. Thereafter, a mid-level trough is expected to create a pronounced weakness in the ridge along 40W-45W longitude. This steering pattern is likely to cause Helene to turn northwestward to north-northwestward during the latter part of the forecast period. The official track forecast is very similar to the previous NHC prediction and lies roughly in the middle of the numerical guidance suite.

FORECAST POSITIONS AND MAX WINDS

INIT  09/1500Z 13.2N  25.0W   55 KT  65 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 12H  10/0000Z 13.7N  27.1W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 24H  10/1200Z 14.3N  30.0W   75 KT  85 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 36H  11/0000Z 14.9N  32.8W   85 KT 100 MPH - Category 2 (ESE Hidden Beach, Bermuda)
 48H  11/1200Z 15.7N  35.2W   90 KT 105 MPH - Category 2 (ESE Hidden Beach, Bermuda)
 72H  12/1200Z 17.8N  38.6W   80 KT  90 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 96H  13/1200Z 21.0N  41.0W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
120H  14/1200Z 26.0N  43.0W   55 KT  65 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)

NWS National Hurricane Center Miami FL 500 AM AST Sun Sep 09 2018

Satellite imagery indicates that Helene continues to become better organized with an increase in banding overnight. A couple of recent microwave images show that the inner core continues to organize with a low-level ring evident and indication that a banding-type eye is forming. Dvorak satellite T-numbers have increased and support an initial intensity of 55 kt.

Helene is forecast to remain over SSTs of 27-28C during the next couple of days. The upper-level wind pattern is also expected to be quite favorable, and these conditions should support steady intensification. Slightly cooler waters and increasing southwesterly shear from an upper-level trough to the northwest of Helene are likely to put an end to the intensification process by day 3. After that time, Helene is forecast to move closer to the aforementioned trough with southwesterly shear and drier mid-level air likely to result in weakening by day 4 and 5. The NHC intensity forecast is near the IVCN intensity consensus model, but it is a little more conservative than the FSSE and HCCA models.

The tropical storm is moving westward or 270 degrees at 12 kt. A deep-layer ridge to the north of the storm should steer Helene westward to west-northwestward during the next couple of days. By mid-week, Helene is expected to approach a weakness in the ridge between 40-50W longitude and the cyclone is predicted to turn northwestward, then north-northwestward by the end of the period. The track guidance is in fairly good agreement on this scenario, but there is some spread later in the period as to exactly what longitude the northwesterly turn takes place. The NHC forecast is between the various dynamical model solutions and is close to the HFIP corrected consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  09/0900Z 13.2N  24.0W   55 KT  65 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 12H  09/1800Z 13.4N  26.0W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 24H  10/0600Z 14.0N  28.9W   75 KT  85 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 36H  10/1800Z 14.7N  31.9W   85 KT 100 MPH - Category 2 (ESE Hidden Beach, Bermuda)
 48H  11/0600Z 15.4N  34.5W   90 KT 105 MPH - Category 2 (ESE Hidden Beach, Bermuda)
 72H  12/0600Z 17.4N  38.3W   80 KT  90 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 96H  13/0600Z 20.5N  41.0W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
120H  14/0600Z 25.0N  42.5W   50 KT  60 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)

NWS National Hurricane Center Miami FL 800 PM AST Sat Sep 08 2018

DISCUSSION AND OUTLOOK

At 800 PM AST (0000 UTC), the center of Tropical Storm Helene  was located near latitude 13.6 North, longitude 22.2 West. Helene is moving toward the west near 13 mph (20 km/h), and this general motion will likely continue through Sunday. A gradual turn toward the west-northwest with an increase in forward speed is expected Sunday night through Tuesday. On the forecast track, Helene will pass very close to the southern Cabo Verde Islands tonight and early Sunday.

Maximum sustained winds are near 60 mph (95 km/h) with higher gusts. Strengthening is expected during the next few days, and Helene is forecast to become a hurricane on Sunday.

Tropical-storm-force winds extend outward up to 60 miles (95 km) from the center. The estimated minimum central pressure is 998 mb (29.47 inches).

HAZARDS AFFECTING LAND

WIND: Tropical storm conditions are expected to first reach the warning area during the next several hours. Hurricane conditions are possible in the southernmost Cabo Verde Islands on Sunday.

RAINFALL: Helene is expected to produce total rain accumulations of 3 to 5 inches over the southern Cabo Verde Islands through Sunday, with isolated maximum amounts of 8 inches. This rainfall may produce life-threatening flash floods.

NWS National Hurricane Center Miami FL 500 PM AST Sat Sep 08 2018

During the past few hours, Helene  has developed a convective band that wraps about three-quarters of the way around the storm, and the intensity of the inner core convection has increased. In addition, microwave imagery shows an elliptical inner ring feature. Satellite intensity estimates at 18Z were in the 35-50 kt range, and given the recent increase in organization the initial intensity is increased to 50 kt.

Helene should continue to strengthen through at least 72 h in a light vertical shear environment over sea surface temperatures near 27C. This part of the intensity forecast has been increased over the previous forecast and lies near the intensity consensus. However, there remain a couple of alternate forecast scenarios. The first is that Helene could rapidly intensify and become stronger than currently forecast, and the rapid intensification index of the SHIPS model has about a 35 percent chance of 25 kt of strengthening in the next 24 h. The second is that a large plume of African dust and associated dry air that has spread over the Cabo Verde Islands starts entraining into the cyclone and inhibits intensification. The former alternative is more likely than the latter at this time. After 72 h, Helene is expected to encounter increasing southwesterly shear associated with a mid- to upper-level trough over the central Atlantic, which should cause the system to weaken significantly.

The initial motion is 275/11. There again is little change in the forecast guidance or the forecast track, with Helene expected to move westward to west-northwestward for the next 3 days or so on the south side of the subtropical ridge, then turn northwestward in response to the aforementioned trough.

FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 13.6N  21.7W   50 KT  60 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 12H  09/0600Z 13.7N  23.5W   55 KT  65 MPH - Tropical Storm (ESE Hidden Beach, Bermuda)
 24H  09/1800Z 14.1N  26.2W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 36H  10/0600Z 14.7N  29.1W   75 KT  85 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 48H  10/1800Z 15.5N  32.0W   85 KT 100 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 72H  11/1800Z 17.5N  37.0W   85 KT 100 MPH - Category 1 (ESE Hidden Beach, Bermuda)
 96H  12/1800Z 19.5N  40.5W   65 KT  75 MPH - Category 1 (ESE Hidden Beach, Bermuda)
120H  13/1800Z 23.0N  42.5W   55 KT  65 MPH- Tropical Storm (ESE Hidden Beach, Bermuda)

NWS National Hurricane Center Miami FL 200 PM AST Sat Sep 08 2018

DISCUSSION AND OUTLOOK

At 200 PM AST (1800 UTC), the center of Tropical Storm Helene  was located near latitude 13.4 North, longitude 21.1 West. Helene is moving toward the west near 13 mph (20 km/h), and this general motion will likely continue through Sunday. A gradual turn toward the west-northwest with an increase in forward speed is expected Sunday night through Tuesday. On the forecast track, Helene will pass very close to the southern Cabo Verde Islands tonight and early Sunday.

Maximum sustained winds have increased to near 50 mph (85 km/h) with higher gusts. Satellite imagery indicates that Helene is becoming better organized, and strengthening is forecast for the next couple of days. Helene is expected to become a hurricane on Sunday or Sunday night.

Tropical-storm-force winds extend outward up to 60 miles (95 km) from the center. The estimated minimum central pressure is 1001 mb (29.56 inches).

HAZARDS AFFECTING LAND

WIND: Tropical storm conditions are expected to first reach the warning area tonight or early Sunday. Hurricane conditions are possible in the southernmost Cabo Verde Islands on Sunday.

RAINFALL: Helene is expected to produce total rain accumulations of 3 to 5 inches over the southern Cabo Verde Islands through Sunday, with isolated maximum amounts of 8 inches. This rainfall may produce life-threatening flash floods.

NWS National Hurricane Center Miami FL 1100 AM AST Sat Sep 08 2018

Tropical Storm Helene – The structure of Helene continues to improve, with convective banding wrapping around almost completely around the center of the cyclone in visible satellite imagery. Infrared imagery suggests this convection is not very strong at the moment, though, and this may be why recent scatterometer data indicates that the storm has not strengthened since the last advisory. The initial intensity remains 40 kt based on the scatterometer winds.

All indications are that Helene should strengthen through at least 72 h in a light vertical shear environment over sea surface temperatures near 27C. This part of the intensity forecast is similar to the previous forecast and lies near the intensity consensus. However, there are a couple of alternate forecast scenarios. The first is that Helene could rapidly intensify and become stronger than currently forecast. The second is that a large plume of African dust and associated dry air that has spread over the northern Cabo Verde Islands starts entraining into the cyclone and inhibits intensification. The former alternative seems more likely than the latter at this time, and if RI begins later intensity forecasts will need to be increased. After 72 h, Helene is expected to encounter increasing southwesterly shear associated with a mid- to upper-level trough over the central Atlantic, which should cause the system to weaken.

Helene’s center appears to have again re-formed, this time a little to the south. The somewhat uncertain initial motion is 265/11. Other than a slight nudge to the south due to the initial position, there is little change in the forecast track for Helene. The cyclone should moved westward to west-northwestward for the next 3-4 days on the south side of the subtropical ridge, then turn northwestward in response to the aforementioned trough.

FORECAST POSITIONS AND MAX WINDS

INIT  08/1500Z 13.4N  20.6W   40 KT  45 MPH - Tropical Storm (WSW Dakar, West Africa)
 12H  09/0000Z 13.6N  22.4W   50 KT  60 MPH - Tropical Storm (WSW Dakar, West Africa)
 24H  09/1200Z 13.9N  24.9W   60 KT  70 MPH - Tropical Storm (WSW Dakar, West Africa)
 36H  10/0000Z 14.6N  27.8W   70 KT  80 MPH - Category 1 (São Filipe,Cape Verde)
 48H  10/1200Z 15.3N  30.8W   80 KT  90 MPH - Category 1 (São Filipe,Cape Verde)
 72H  11/1200Z 17.0N  36.0W   80 KT  90 MPH - Category 1 (São Filipe,Cape Verde)
 96H  12/1200Z 19.0N  40.0W   70 KT  80 MPH - Category 1 (São Filipe,Cape Verde)
120H  13/1200Z 22.5N  42.5W   60 KT  70 MPH - Tropical Storm (São Filipe,Cape Verde)

NWS National Hurricane Center Miami FL 500 AM AST Sat Sep 08 2018

Helene’s  structure appears to be quickly improving this morning. The initial intensity has been increased to 40 kt based on a blend of the latest subjective Dvorak intensity estimates and the UW-CIMSS SATCON. Several microwave passes overnight indicate that the cyclone has become more vertically aligned, likely due to a reformation of the center beneath the strongest convection. In fact, a 0733 UTC SSMIS image shows the early development of a low- to mid-level eye, which is often a precursor to intensification.

Nearly all of the intensity guidance seems to have picked up on the improved structure of Helene, and the models show far more intensification than before, especially through 48 h. Consequently, a significant change has been made to the intensity forecast, which now calls for Helene to become a hurricane on Sunday while it is near the Cabo Verde islands. Once the cyclone passes the islands, the environment is expected to remain favorable for intensification for at least 72 h. By the end of the forecast period, all of the guidance shows some weakening due primarily to increased shear associated with an approaching mid- to upper-level trough over the central Atlantic. Given the recent microwave imagery signature, low shear, and warm SSTs, its possible that the current forecast is still too conservative, especially for the first 36 h of the forecast, and further adjustments could be required if it becomes clear that Helene is intensifying at a faster rate than expected.

Smoothing through the possible reformation of the center, Helene appears to be moving west with an initial motion around 275/10 kt. Little change was required to the track forecast, which has been nudged only slightly south for the first 48 h of the forecast, in line with the latest track model consensus. A westward motion should continue for the next couple of days, as Helene is steered by a mid-level ridge to the north. By day 5, the aforementioned trough over the central Atlantic should force Helene to turn toward the northwest.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 13.7N  19.6W   40 KT  45 MPH - Tropical Storm (WSW Dakar, West Africa)
 12H  08/1800Z 13.9N  21.2W   50 KT  60 MPH - Tropical Storm (WSW Dakar, West Africa)
 24H  09/0600Z 14.1N  23.5W   60 KT  70 MPH - Tropical Storm (WSW Dakar, West Africa)
 36H  09/1800Z 14.7N  26.3W   70 KT  80 MPH - Category 1 (WSW Dakar, West Africa)
 48H  10/0600Z 15.4N  29.3W   75 KT  85 MPH - Category 1 (WSW Dakar, West Africa)
 72H  11/0600Z 17.2N  35.1W   80 KT  90 MPH - Category 1 (WSW Dakar, West Africa)
 96H  12/0600Z 19.0N  39.5W   70 KT  80 MPH - Category 1 (WSW Dakar, West Africa)
120H  13/0600Z 22.0N  42.5W   65 KT  75 MPH - Category 1 (WSW Dakar, West Africa)

NWS National Hurricane Center Miami FL 1100 PM AST Fri Sep 07 2018

A very timely ASCAT pass indicated that the winds associated with the depression have increased to 35 kt, and also that the center was a little east of the location previously indicated. This is very common in systems during the formative stage. Based on the ASCAT data, the system has been upgraded to Tropical Storm Helene, the eighth named storm of the season. The satellite presentation has also improved during the past several hours, and now the cyclone has large cyclonically curved convective bands to the south of the center. The outflow is fair in all quadrants.

Helene will be moving over warm waters and within an environment of light shear through the next 3 to 4 days, and most of the guidance responds to that environment by gradually strengthening the cyclone. The NHC forecast follows the trend of the intensity consensus, and brings Helene to hurricane intensity in about 3 days.

Currently, Helene is embedded within a southwest monsoon-type flow, and is moving toward the west at about 9 or 10 knots. However, as the cyclone moves away from the African coast, it will become steered by the easterly flow around the subtropical ridge and should then increase in forward speed. Most of the track models are in extremely good agreement, at least for the next 3 days when the confidence in the forecast is high. At the long range, a mid-level trough is expected to develop over the central Atlantic, forcing the cyclone to turn more to the northwest and even north later on. The NHC forecast is in the middle of the guidance envelope and basically on top of the corrected consensus HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 13.6N  18.5W   35 KT  40 MPH - Tropical Storm (WSW Dakar, West Africa)
 12H  08/1200Z 13.8N  19.9W   35 KT  40 MPH - Tropical Storm (WSW Dakar, West Africa)
 24H  09/0000Z 14.3N  22.3W   40 KT  45 MPH - Tropical Storm (WSW Dakar, West Africa)
 36H  09/1200Z 14.8N  25.0W   50 KT  60 MPH - Tropical Storm (WSW Dakar, West Africa)
 48H  10/0000Z 15.4N  28.0W   55 KT  65 MPH - Tropical Storm (WSW Dakar, West Africa)
 72H  11/0000Z 17.0N  33.9W   65 KT  75 MPH - Category 1 (WSW Dakar, West Africa)
 96H  12/0000Z 18.5N  39.0W   65 KT  75 MPH - Category 1 (WSW Dakar, West Africa)
120H  13/0000Z 20.5N  42.5W   60 KT  70 MPH - Tropical Storm (WSW Dakar, West Africa)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Sep 07 2018

Satellite imagery and surface observations indicate that the area of low pressure that moved off the west coast of Africa has developed a closed circulation that appears to be well defined. However, the convective organization, given Dvorak classifications of only T1.0, do not support calling the system a tropical cyclone at this point. However, given that the system is expected to be a tropical storm near the southern Cabo Verde Islands in 36 to 48 hours, advisories are being initiated on this system as a potential tropical cyclone at this time, and a tropical storm warning has been issued for those islands.

Environmental conditions of light to moderate easterly shear and SSTs of 27-28C should support development of this system, and the NHC forecast shows it becoming a tropical storm by 12 hours. Gradual strengthening is indicated by the bulk of the intensity guidance through the next 3-4 days. After that time, the shear is forecast to increase and the system will be moving into somewhat cooler waters, so some weakening is expected by day 5. The NHC forecast is close to the latest HCCA guidance and a little higher than the IVCN consensus.

The initial motion estimate is a somewhat uncertain 275/09 given that the surface circulation is just forming. The synoptic pattern over the eastern Atlantic features a ridge to the north of the disturbance, which is expected to weaken by the end of the period. This steering pattern should result in a westward to west- northwestward motion for the next 72 hours with an increase in forward speed, followed by a decrease in forward speed and a slightly more poleward motion as the system responds to the weakness in the ridge. The initial NHC track forecast is close to the latest HCCA track consensus aid and near the middle of the guidance envelope.

ORECAST POSITIONS AND MAX WINDS

INIT  07/1500Z 13.1N  17.8W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  08/0000Z 13.3N  18.6W   35 KT  40 MPH - Tropical Storm (WSW Dakar, West Africa)
 24H  08/1200Z 13.8N  20.3W   40 KT  45 MPH - Tropical Storm (WSW Dakar, West Africa)
 36H  09/0000Z 14.2N  22.6W   45 KT  50 MPH - Tropical Storm (WSW Dakar, West Africa)
 48H  09/1200Z 14.6N  25.4W   50 KT  60 MPH - Tropical Storm (WSW Dakar, West Africa)
 72H  10/1200Z 16.0N  32.0W   55 KT  65 MPH - Tropical Storm (WSW Dakar, West Africa)
 96H  11/1200Z 18.0N  37.0W   60 KT  70 MPH - Tropical Storm (WSW Dakar, West Africa)
120H  12/1200Z 20.0N  41.0W   55 KT  65 MPH - Tropical Storm (WSW Dakar, West Africa)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Sep 07 2018

DISCUSSION AND OUTLOOK

At 1100 AM AST (1500 UTC), the disturbance was centered near latitude 13.1 North, longitude 17.8 West. The system is moving toward the west near 10 mph (17 km/h). A westward to west- northwestward motion with a gradual increase in forward speed is expected during the next 72 hours. On the forecast track, the disturbance is expected to move near the southern Cabo Verde Islands on Sunday.

Maximum sustained winds are near 35 mph (55 km/h) with higher gusts. Strengthening is forecast during the next 72 hours, and the system is expected to become a tropical depression or tropical storm later today.

NWS National Hurricane Center Miami FL 1100 AM AST Fri Sep 07 2018

DISCUSSION AND OUTLOOK

At 1100 AM AST (1500 UTC), the disturbance was centered near latitude 13.1 North, longitude 17.8 West. The system is moving toward the west near 10 mph (17 km/h). A westward to west- northwestward motion with a gradual increase in forward speed is expected during the next 72 hours. On the forecast track, the disturbance is expected to move near the southern Cabo Verde Islands on Sunday.

Maximum sustained winds are near 35 mph (55 km/h) with higher gusts. Strengthening is forecast during the next 72 hours, and the system is expected to become a tropical depression or tropical storm later today. Conditions are conducive for the disturbance to become a tropical depression or tropical storm later today.

Conditions are conducive for the disturbance to become a tropical depression or tropical storm later today. * Formation chance through 48 hours…high…90 percent * Formation chance through 5 days…high…90 percent

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