Tropical Storm Isaac

Tropical Storm Isaac Reforming 0800 Hours September 17 2018
Tropical Storm Isaac Reforming 0800 Hours September 17 2018

NWS National Hurricane Center Miami FL 800 AM EDT Mon Sep 17 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Tropical Depression Joyce, located a few hundred miles south-southwest of the Azores. The NOAA Weather Prediction Center is issuing advisories on Tropical Depression Florence, located inland over the Appalachian mountains. (see 3 videos below)

1. A weak area of low pressure, associated with the remnants of Isaac, is located just south of Jamaica. Showers and thunderstorms are currently limited, and any development should be slow to occur during the next day or so. By Wednesday, environmental conditions are forecast to become less conducive for re-development to occur when the system moves toward the Yucatan Peninsula. Regardless of development, locally heavy rains and gusty winds are possible over portions of Hispaniola, Jamaica, and eastern Cuba during the next couple of days while the system moves westward to west-northwestward across the northwestern Caribbean Sea. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low…10 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Sun Sep 16 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center has issued the final advisory on Helene that has become a Post-Tropical Cyclone over the northeastern Atlantic Ocean. The National Hurricane Center is issuing advisories on recently downgraded Tropical Depression Joyce, located several hundred miles west-southwest of the Azores. The Weather Prediction Center is issuing advisories on Tropical Depression Florence, located inland over South Carolina.

1. A broad area of low pressure, associated with the remnants of Isaac, is located about 125 miles east-southeast of Kingston, Jamaica. This system continues to produce disorganized showers and thunderstorms over the central Caribbean Sea. Some slow development is possible during the next day or so as it moves west-northwestward toward the northwestern Caribbean Sea. Afterward, environmental conditions are forecast to become less conducive and re-development appears unlikely. Regardless of development, locally heavy rains and gusty winds are forecast over portions of Hispaniola, Jamaica, and eastern Cuba during the next couple of days. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low...20 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Sun Sep 16 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Tropical Depression Florence, located inland over central South Carolina, on Tropical Storm Helene, located a few hundred miles north of the central Azores, and on Tropical Storm Joyce, located about 600 miles west-southwest of the Azores.

1. A broad area of low pressure, associated with the remnants of Isaac , is located a couple of hundred miles southeast of Kingston, Jamaica. This system is producing an area of disorganized showers and thunderstorms over the north-central Caribbean Sea. Some slow development is possible during the next day or so as it moves west-northwestward toward the northwestern Caribbean Sea. Afterward, environmental conditions are forecast to become less conducive and re-development appears unlikely. Regardless of development, locally heavy rains and gusty winds are forecast over portions of Hispaniola, Jamaica, and eastern Cuba during the next couple of days. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low…20 percent

NWS National Hurricane Center Miami FL 200 PM EDT Sat Sep 15 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Tropical Storm Florence, located inland over extreme eastern South Carolina, on Tropical Storm Helene, located about 100 miles west of the western Azores, and on Tropical Storm Joyce, located about 900 miles west-southwest of the Azores.

The remnants of Tropical Storm Issac , now a tropical wave, are located over the central Caribbean Sea. Slow re-development of this system is possible during the next couple of days as it moves west-northwestward into the northwestern Caribbean Sea. After that time, upper-level winds are expected to become less conducive for re-development. Locally heavy rains and gusty winds are forecast over portions of Hispaniola, Jamaica, and eastern Cuba during the next couple of days. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low…20 percent.

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

A NOAA Hurricane Hunter aircraft investigated Isaac  over the past few hours and was unable to identify a well-defined center. The plane found multiple small and poorly-defined circulations embedded within a larger area of weak and variable winds, well to the south of the previous estimated center location. The plane also found that the minimum pressure has risen to around 1006 mb. All of this data confirmed what two late-arriving ASCAT overpasses from earlier this evening indicated, that Isaac has opened up into a southwest to northeast oriented trough. Since Isaac does not have a well-defined center, it no longer meets the NHC definition of a tropical cyclone and this is the last advisory.

The strongest SFMR winds found by the plane were around 30 kt, mainly to the north of the deep convection, so the intensity has been set at that value. Although some of the regional hurricane models suggest that small short-lived circulations like the ones found by the plane could temporarily spin up within the larger envelope of the tropical wave over the next several days, none of the dynamical models currently show any significant redevelopment of Isaac. The wave will likely continue westward across the Caribbean Sea over the next few days, producing gusty winds and occasional heavy rains over portions of Hispaniola and Jamaica.

This is the last public advisory issued by the National Hurricane Center on Isaac. Additional information can be found in High Seas Forecasts issued by the National Weather Service.

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

Isaac  has rapidly lost organization since the last advisory. The cyclone has been devoid of any deep convection for most of the evening, with only a small burst recently forming well to the west of where Isaac’s low-level center is believed to be located. While it is possible, if not likely, that the maximum winds have decreased a little since this afternoon due to the lack of convection, the initial intensity has been held at 35 kt based on earlier aircraft data. A NOAA Hurricane Hunter aircraft is scheduled to investigate Isaac again overnight and will provide a better estimate of the strength and status of Isaac.

The global models continue to insist that Isaac could open into a trough at any time, and based on the current state of the cyclone, this could be imminent. The regional hurricane models, however, show the development of one or more mesoscale lows embedded within a larger cyclonic envelope that cause Isaac to persist for at least a few more days in those models. While the global model solution still seems more likely, I can’t rule out that Isaac will persist a little longer, so the NHC forecast still carries a 72 h forecast point. Regardless of whether Isaac is a tropical storm or a tropical wave over the next few days, heavy rains and occasional gusty winds are still likely across much of the Greater Antilles through the next couple of days.

It has been very difficult to try to pinpoint a low-level center of Isaac over the past few hours, but based on microwave data it appears to still be moving west at about 13 kt. The GFS and ECMWF are in very good agreement and show Isaac, or its remnants, moving steadily westward for the next several days. The regional models on the other hand show a farther north motion, in part due to one or more reformations of the center. The NHC forecast continues to favor the global model solutions and is on the south side of the guidance envelope, however if it becomes clear that Isaac will survive longer, a northward adjustment may be required in later advisories.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0300Z 15.4N  69.5W   35 KT  40 MPH - Tropical Storm (ESE Santo Domingo, Dominican Republic)
 12H  15/1200Z 15.5N  71.3W   35 KT  40 MPH - Tropical Storm (WSW Barahona, Dominican Republic)
 24H  16/0000Z 15.7N  73.3W   35 KT  40 MPH - Tropical Storm (WSW Jacmel, Haiti)
 36H  16/1200Z 16.2N  75.3W   35 KT  40 MPH - Tropical Storm (ESE Morant Bay, Jamaica)
 48H  17/0000Z 16.7N  77.4W   30 KT  35 MPH - Tropical Storm (WSW Hayes, Jamaica)
 72H  18/0000Z 18.7N  81.4W   30 KT  35 MPH - Tropical Storm (WSW George Town, Cayman Islands)
 96H  19/0000Z...DISSIPATED

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

Tropical Storm Isaac  – It has been a very interesting flight of the NOAA Hurricane Hunter aircraft this afternoon. Dropsonde and SFMR data indicate that the maximum winds are around 35 kt again, and the pressure has fallen to about 1002 mb. However, the system is tilted southward with vertical height due to northerly shear, and scatterometer and Hurricane Hunter tail-Doppler radar data indicate the system is elongated from northeast to southwest, albeit closed. Note that the plane was unable to sample the southwestern quadrant well because of strong convection and intense lightning, so the flight-level winds showing no circulation were unrepresentative.

The intensity forecast hasn’t gotten any easier with Isaac. Vertical shear is expected to decrease a bit more, which might diminish the effects of dry air seen on water vapor images around the cyclone. However, the GFS and ECMWF continue to insist that Isaac will degenerate into a wave within the next 2 or 3 days, which is almost unanimously supported by their respective ensemble members. This is the solution that the official forecast follows. It should be noted that none of the global models show pressures this low now with Isaac, except the UKMET, and that model is the one that holds onto the circulation the longest. The reality of the situation is that the long-term future of Isaac is cloudy, and the best advice is just to follow NHC advisories every 6 hours to see if we gain any confidence.

Isaac is moving westward at about 12 kt. A ridge over the southwestern Atlantic should steer the cyclone westward over the weekend then west-northwestward early next week as Isaac reaches a weaker portion of the ridge. Just like yesterday, a stronger system would likely turn more to the west-northwest earlier, moving closer to Jamaica. Since the system is forecast to be weaker, the forecast lies on the westward and faster side of the guidance, and should also be considered low confidence since it depends on the intensity. Interests in Jamaica should monitor the progress of the Isaac.

FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 15.3N  68.2W   35 KT  40 MPH - Tropical Storm (WSW Ponce, Puerto Rico)
 12H  15/0600Z 15.4N  70.1W   35 KT  40 MPH - Tropical Storm (WSW Santo Domingo, Dominican Republic)
 24H  15/1800Z 15.4N  72.2W   35 KT  40 MPH - Tropical Storm (ESE Jacmel, Haiti)
 36H  16/0600Z 15.7N  74.2W   35 KT  40 MPH - Tropical Storm (WSW Les Cayes, Haiti)
 48H  16/1800Z 16.2N  76.3W   30 KT  35 MPH - Tropical Storm (S Morant Bay, Jamaica)
 72H  17/1800Z 17.5N  80.0W   25 KT  30 MPH - Tropical Storm (WSW Negril, Jamaica)
 96H  18/1800Z...DISSIPATED

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

Deep convection has been increasing near the center of Isaac , but visible imagery indicates that the circulation has become more elongated from northeast to southwest. It is possible that Isaac is in the process of losing a well-defined center, but we will know more about the wind structure in a few hours since the NOAA Hurricane Hunter aircraft should be in the area. The initial wind speed is held at 30 kt.

Vertical shear has decreased near Isaac, with some outflow in the western semicircle noted and consolidation of the convection. Still, this area of the eastern Caribbean isn’t known as the graveyard for nothing, and the system might just be too shallow and disorganized to take advantage of the changing environment. Almost all of the dynamical models, save the UKMET, show the cyclone moving westward to west-northwestward at a slower forward speed during the next few days and degenerating into an open wave in the central Caribbean Sea. This solution is also supported by fewer members of the GFS and ECMWF ensemble now maintaining a coherent system in the long range. Little change is made to the previous track and intensity forecasts, and we will see what the aircraft finds this afternoon and if the 1200 UTC model cycle maintains the dissipation trend.

FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 14.9N  67.3W   30 KT  35 MPH - Tropical Storm (ESE Roseau, Dominica)
 12H  15/0000Z 15.0N  69.0W   30 KT  35 MPH - Tropical Storm (WSW Ponce, Puerto Rico)
 24H  15/1200Z 15.1N  71.3W   30 KT  35 MPH - Tropical Storm (WSW Santo Domingo, Dominican Republic)
 36H  16/0000Z 15.2N  73.3W   25 KT  30 MPH - Tropical Storm (WSW Les Cayes, Haiti)
 48H  16/1200Z 15.6N  75.3W   25 KT  30 MPH - Tropical Storm (ESE Kingston, Jamaica)
 72H  17/1200Z 17.0N  79.5W   25 KT  30 MPH - Tropical Storm (ESE Treasure Beach, Jamaica)
 96H  18/1200Z...DISSIPATED

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

The center of Isaac  has been mostly exposed during the past several hours as temporary bursts of convection fire just east of the center. Assuming some spin down of the circulation after the plane left, the initial wind speed is set to 35 kt. Since there haven’t been any reports of sustained tropical-storm-force winds in the Lesser Antilles and the center is moving away, all watches and warning have been lowered. However, there remains a threat of flash flooding due to a large area of rain that has yet to move through the Lesser Antilles.

Strong shear and mid-level dry air is forecast to persist for the next day or so, which should cause Isaac to drop back to tropical depression status on Friday. However, the shear is forecast to weaken by the weekend, and moisture values are expected to rise as the system gets deeper into the Caribbean Sea. These conditions would normally allow for some restregthening, but the initial character of the system is pretty weak, and it is uncertain whether there will be much left of Isaac to take advantage of the more conducive conditions. Model guidance has been inconsistent on the future of Isaac, which speaks to the lack of predictability of this situation. The bulk of the guidance show Isaac staying weak or degenerating into a tropical wave over the next few days, so the official forecast follows that consensus. At this point it is just best to keep an eye on the long-range forecast on each advisory to see if there are any changes.

Isaac is moving westward at about 15 kt. A mid-level ridge is forecast to persist over the southwestern Atlantic, steering Isaac westward for the next few days, with a turn to the west-northwest possible at long range. The future track is very uncertain at long range because it is dependent on the intensity. If Isaac degenerates into a wave, the remnants will likely move south and west of the guidance mean, and if the cyclone is stronger, it will probably move slower and more to the right. The new NHC forecast assumes that Isaac will stay weak, and therefore is on the left and faster side of the guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  13/2100Z 14.9N  63.2W   35 KT  40 MPH - Tropical Storm (WSW Roseau, Dominca)
 12H  14/0600Z 14.9N  65.4W   35 KT  40 MPH - Tropical Storm (WSW Roseau, Dominca)
 24H  14/1800Z 14.9N  68.2W   30 KT  35 MPH - Tropical Storm (WSW Ponce, Puerto Rico)
 36H  15/0600Z 14.9N  70.8W   30 KT  35 MPH - Tropical Storm (WSW Barahona, Dominican Republic)
 48H  15/1800Z 15.0N  73.0W   30 KT  35 MPH - Tropical Storm (SW Les Cayes, Haiti)
 72H  16/1800Z 16.2N  77.4W   30 KT  35 MPH - Tropical Storm (WSW Hayes, Jamaica)
 96H  17/1800Z 17.5N  81.5W   30 KT  35 MPH - Tropical Storm (SW George Town, Cayman Islands)
120H  18/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 200 PM AST Thu Sep 13 2018

DISCUSSION AND OUTLOOK

At 200 PM AST (1800 UTC), the center of Tropical Storm Isaac  was located by satellite images near latitude 14.9 North, longitude 62.5 West. Isaac is moving toward the west near 18 mph (30 km/h). A westward track with a slower forward motion is forecast to continue for the next few days. On the forecast track, Isaac should move farther away from Lesser Antilles today, and then move across the eastern and central Caribbean Sea through the weekend.

Maximum sustained winds remain near 45 mph (75 km/h) with higher gusts. Gradual weakening is forecast over the next few days as Isaac moves through the Caribbean Sea.

Tropical-storm-force winds extend outward up to 160 miles (260 km) from the center.

The estimated minimum central pressure is 1006 mb (29.71 inches).

HAZARDS AFFECTING LAND

RAINFALL: Tropical Storm Isaac is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up to 8 to 10 inches over Dominica. Rainfall amounts of 2 to 4 inches with isolated amounts up to 6 inches over elevated terrain are forecast across Martinique and Guadeloupe. Rainfall totals of 2 to 4 inches are forecast across southeastern Puerto Rico, while totals of 1 to 2 inches with maximum amounts of 3 inches are possible across the remainder of Puerto Rico and the U.S. Virgin Islands. This rainfall may cause dangerous flash flooding.

WIND: Tropical storm conditions are occuring within portions of the warning area and should continue through the afternoon hours. Tropical storm conditions are possible within the watch area today.

SURF: Swells generated by Isaac are affecting portions of the Lesser Antilles. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

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

Satellite imagery indicate that Isaac remains a poorly organized tropical cyclone, with the low-level center mostly exposed during the past several hours. The center of Isaac passed between Dominica and Martinique around 1300 UTC, with westerly winds reported on both St. Lucia and Martinique, indicating that Isaac is still a tropical storm. Data from a NOAA P-3 Hurricane Hunter aircraft indicate that maximum winds remain near 40 kt, although the area of tropical- storm-force winds has shrunk since yesterday.

1200 UTC radiosonde data from the eastern Caribbean and Hurricane Hunter aircraft data indicate that the circulation of Isaac is fairly shallow and small. It is entirely possible that Isaac will degenerate into a tropical wave during the next day or two due to the continued effects of strong shear. However, the environment becomes much more conducive for increased organization in a few days when the cyclone, or its remnants, moves across the west-central Caribbean Sea. Model guidance is not consistent at all on whether Isaac will survive as a tropical cyclone, which frankly might not be a very predictable event given the initial uncertainties. The best course of action is to let the full 1200 UTC guidance suite ingest all of the latest radiosonde and other data before making any changes, so this advisory holds the status quo for now.

Isaac is moving a little south-of-west and a bit faster, since it was devoid of convection most of night, at about 265/17. A mid-level ridge is forecast to persist over the southwestern Atlantic, steering Isaac westward for the next few days. The track forecast is fairly dependent on the intensity forecast, since a deeper tropical cyclone will move slower due to slightly weaker winds in the middle part of the atmosphere. The forecast has been too slow so far, and with the current forecast showing little strengthening, the official track prediction is shifted west of the previous one, close to the NOAA corrected consensus aid. At the end of the forecast, if Isaac remains a tropical cyclone, it could start moving more toward the west-northwest, but I would place little emphasis on the 96-hour point until we get a better handle on the future structure of Isaac.

Tropical Storm Isaac Almost on Land

Key Messages:

1. Isaac is expected to produce tropical-storm-force winds across portions of the Lesser Antilles into this afternoon, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe.

2. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, Barbuda, Saba and St. Eustatius, St. Maarten, and St. Martin. Tropical storm conditions are possible on these islands today and interests on those islands should follow any advice given by their local officials.

FORECAST POSITIONS AND MAX WINDS

INIT  13/1500Z 14.9N  61.8W   40 KT  45 MPH - Tropical Storm (WSW Rosalie, Dominca)
 12H  14/0000Z 14.9N  64.2W   35 KT  40 MPH - Tropical Storm (WSW Rosalie, Dominca)
 24H  14/1200Z 14.9N  67.0W   35 KT  40 MPH - Tropical Storm (SW Ponce, Puerto Rico)
 36H  15/0000Z 14.9N  69.8W   35 KT  40 MPH - Tropical Storm (SW Santo Domingo, Dominican Republic)
 48H  15/1200Z 15.0N  72.3W   35 KT  40 MPH - Tropical Storm (S Jacmel, Haiti)
 72H  16/1200Z 15.5N  76.5W   35 KT  40 MPH - Tropical Storm (S Kingston, Jamaica)
 96H  17/1200Z 17.0N  80.5W   30 KT  35 MPH - Tropical Storm (SE George Town, Cayman Islands)
120H  18/1200Z...DISSIPATED

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

Tropical Storm Isaac Data from a NOAA P-3 Hurricane Hunter aircraft that departed the storm shortly before 0600 UTC indicated that the tropical storm has weakened a little more. Maximum flight-level winds were around 51 kt, while maximum SFMR winds were only 41 kt. Both of these support an initial intensity of 40 kt. Wind measurements from the plane also suggested that Isaac is close to opening up into a tropical wave and its circulation is likely barely closed at the surface. Another P-3 mission is scheduled for this morning.

Isaac is nearly completely devoid of deep convection at the moment, and is not trackable in radar data from Guadeloupe and Martinique. Given the lack of deep convection to sustain the cyclone, continued gradual weakening is expected. However, given the extent of 40 mph winds observed by the NOAA plane, Isaac is still likely to bring gusty conditions to many of the Leeward Islands through this afternoon. The dynamical guidance is now in fairly good agreement on the intensity of Isaac for the next few days, and nearly all of the models depict Isaac opening into a trough between 48 and 96 h. The NHC forecast conservatively maintains Isaac through day 4 in an attempt to maintain some continuity, but it is very possible that Isaac will open into a trough sooner than indicated. Some of the dynamical models suggest that regeneration could occur in the western Caribbean, however the predictability of such an event is too low to explicitly show in the forecast at this point.Tropical Storm Isaac Tropical Force Winds 0200 Hours September 12 2018

Isaac is still moving quickly westward, with an initial motion of 275/15 kt. Confidence in the track forecast remains high, and the track guidance is tightly clustered through the forecast period. Very little change was made to the track forecast, which continues to closely follow the HCCA model. As long as Isaac remains a shallow cyclone, it should continue to be steered westward by the low-level tradewind flow, even if, or when, it opens up into a trough.

Key Messages:

1. Isaac is expected to produce tropical-storm-force winds across portions of the Lesser Antilles this morning and afternoon, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe.

2. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, Barbuda, Saba and St. Eustatius, St. Maarten, and St. Martin. Tropical storm conditions are possible on these islands today and interests on those islands should follow any advice given by their local officials.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 15.4N  59.7W   40 KT  45 MPH - Tropical Storm (WSW Rosalie, Dominca)
 12H  13/1800Z 15.5N  61.8W   35 KT  40 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 24H  14/0600Z 15.5N  64.8W   35 KT  40 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 36H  14/1800Z 15.5N  67.8W   35 KT  40 MPH - Tropical Storm (WSW Ponce, Puerto Rico)
 48H  15/0600Z 15.4N  70.4W   35 KT  40 MPH - Tropical Storm (WSW San Cristobal, Dominican Republic)
 72H  16/0600Z 15.6N  74.8W   35 KT  40 MPH - Tropical Storm (WSW Les Anglais, Haiti)
 96H  17/0600Z 16.9N  78.5W   30 KT  35 MPH - Tropical Storm (SW Negril, Jamaica)
120H  18/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Wed Sep 12 2018

GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter data show that the center of Isaac has become fully exposed on the northwestern side of a shrinking area of deep convection. There is still a lot of wind there, however, with the NOAA plane observing SFMR and adjusted flight-level winds to support 50 kt as an initial intensity.

An upper-level trough is dropping southward over the central Atlantic, which has increased the shear this morning over Isaac. This high shear is forecast to remain for the next day or two, and it is possible that Isaac will degrade into a tropical wave during that time, although the area of tropical-storm-force winds will likely not go away for a while. In a couple of days, while the wind shear could decrease over the east-central Caribbean Sea, there might not be much of a system left to take advantage of the more conducive conditions. As a compromise, the official forecast shows a slow weakening, in the direction of the model consensus, and continues to show dissipation after 96 hours in line with the GFS/ECMWF guidance. I wouldn’t pay much attention to the long range forecast for now, given the uncertainties in Isaac surviving during the next day or two.

The low-level center of Isaac has recently accelerated, moving westward at 15 kt, as it decoupled from the slower mid-level flow. Assuming the system stays vertically coherent, a large mid-tropospheric high is forecast to persist for the next several days. This should keep the system moving westward at a somewhat slower pace throughout the forecast period. There isn’t much spread in the guidance, and the official forecast is close to the previous one after accounting for the initial position. The long-range track forecast highly depends on the intensity forecast, so it should be considered low confidence.

Key Messages:

1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials.

2. Life-threatening flash flooding is possible with Isaac. The storm is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up 8 inches across Martinique, Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with isolated amounts to 4 inches are expected across Puerto Rico and the southern United States Virgin Islands.

FORECAST POSITIONS AND MAX WINDS

INIT  12/1500Z 15.0N  54.7W   50 KT  60 MPH - Tropical Storm (ESE Rosalie, Dominica)
 12H  13/0000Z 15.1N  56.7W   50 KT  60 MPH - Tropical Storm (E Rosalie, Dominica)
 24H  13/1200Z 15.3N  59.4W   45 KT  50 MPH - Tropical Storm (E Rosalie, Dominica)
 36H  14/0000Z 15.3N  62.6W   45 KT  50 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 48H  14/1200Z 15.4N  65.6W   40 KT  45 MPH - Tropical Storm (W Portsmouth, Dominica)
 72H  15/1200Z 15.5N  71.0W   40 KT  45 MPH - Tropical Storm (ESE Barahona, Dominican Republic)
 96H  16/1200Z 15.5N  75.0W   35 KT  40 MPH - Tropical Storm (ESE Morant Bay, Jamaica)
120H  17/1200Z...DISSIPATED

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

Isaac is a poorly organized tropical storm. Geostationary satellite and microwave images indicate that the center of the cyclone is located near the western edge of the main area of deep convection. This asymmetric cloud pattern is due to about 20 kt of west-northwesterly shear. The satellite intensity estimates have been decreasing, and an average of the Dvorak CI-numbers supports lowering the initial wind speed slightly to 50 kt. The NOAA Hurricane Hunters are scheduled to investigate Isaac later this morning, and that data should provide a better assessment of the storm’s intensity and structure.

Isaac continues to move due westward at about 13 kt. The track forecast reasoning remains unchanged. A fairly strong mid-level ridge to the north of Isaac should steer the system westward at about the same forward speed during the next several days. The models are now in fairly good agreement, and the NHC track forecast is just a little north of the previous one to come in line with the latest guidance. Based on this forecast, Isaac is expected to cross portions of the Leeward and Windward Islands in about 36 hours.Tropical Storm Isaac Tropical Force Winds 0200 Hours September 12 2018

Isaac is feeling the effects of wind shear, and the global models suggest that the shear will likely remain moderate to strong during the next couple of days. These hostile winds combined with nearby dry air will likely cause Isaac to gradually weaken during the next several days. The intensity models are in fair agreement, and this forecast is close to the model consensus, except a little lower than that aid at 96 hours. The latest runs of the GFS and ECMWF models show Isaac opening up into a trough by day 5, and the NHC forecast follows those models.

Key Message:

1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0900Z 14.5N  53.5W   50 KT  60 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 12H  12/1800Z 14.8N  55.7W   50 KT  60 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 24H  13/0600Z 15.0N  58.6W   45 KT  50 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 36H  13/1800Z 15.2N  61.5W   45 KT  50 MPH - Tropical Storm (W Scotts Head, Dominica)
 48H  14/0600Z 15.4N  64.8W   40 KT  45 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 72H  15/0600Z 15.3N  70.6W   40 KT  45 MPH - Tropical Storm (ESE Barahona, Dominican Republic)
 96H  16/0600Z 15.2N  75.0W   35 KT  40 MPH - Tropical Storm (Kingston, Jamaica)
120H  17/0600Z...DISSIPATED

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

A slew of recent microwave passes have revealed that Tropical Storm Isaac’scenter is outrunning the deep convection by about a degree due to strengthening westerly shear. With the degradation in structure, Dvorak estimates have fallen to T3.0/3.5 from both TAFB and SAB. The initial intensity is being set at 55 kt to match the CI number (3.5), but I wouldn’t be surprised if the actual intensity is closer to the final-T number.

Isaac is moving quickly westward, or 270/14 kt. Ridging to the north should keep the cyclone on a similar speed and path for much of the forecast period. The only main difference among the track models is Isaac’s future speed, and the NHC official forecast continues to favor the faster models–the GFS, ECMWF, and HCCA models. Since the forecast thinking is unchanged, the updated NHC forecast is not too different from the previous one. Isaac’s center is expected to move across the Lesser Antilles near Martinique and Dominica on Thursday.

Vertical shear is expected to increase and turn more northwesterly during the next 36 hours as Isaac moves closer to the base of an upper-level trough which extends northeast of the Leeward Islands. Therefore, deep convection is expected to remain decoupled from the low-level center, and Isaac’s structure could degrade further over the next day or two. The cyclone’s maximum winds are also expected to decrease, and the NHC official forecast is close to the HCCA, Florida State Superensemble, and ICON intensity consensus aids. Even though the statistical-dynamical models and the HWRF restrengthen Isaac by days 4 and 5 when the shear decreases, there may not be much left of the cyclone for any strengthening to occur. Both the GFS and ECMWF global models show Isaac weakening to a remnant low or opening up into a trough by days 4 or 5, which is the scenario favored by the NHC official forecast. As such, dissipation is now shown in the forecast at day 5.

Key Message:

1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings have been issued for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0300Z 14.5N  52.3W   55 KT  65 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 12H  12/1200Z 14.6N  54.7W   50 KT  60 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 24H  13/0000Z 14.7N  57.4W   50 KT  60 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 36H  13/1200Z 14.9N  60.4W   45 KT  50 MPH - Tropical Storm (ENE Sainte-Anne, Martinique)
 48H  14/0000Z 15.1N  63.7W   45 KT  50 MPH - Tropical Storm (ESE Roseau, Dominica)
 72H  15/0000Z 15.2N  69.7W   40 KT  45 MPH - Tropical Storm (S Santo Domingo, Dominican Republic)
 96H  16/0000Z 15.0N  74.5W   35 KT  40 MPH - Tropical Storm (ENE Barranquilla, Colombia)
120H  17/0000Z...DISSIPATED

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

Microwave and visible satellite data show that the center of Tropical Isaac   is partially exposed on the western edge of the central dense overcast. Perhaps Isaac is a little weaker than earlier, but a blend of the latest subjective and objective estimates still gives an uncertain initial wind speed of about 60 kt. We should have a better idea of the maximum winds by tomorrow morning when a NOAA Hurricane Hunter enters the storm, or if we get lucky with a scatterometer pass this evening.

Isaac is barely holding on in the face of increasing shear, which is much more evident in the 300-500 mb layer than a conventional analysis at 200 mb. However, convection could redevelop near the center while the storm moves over warming SSTs into a more moist and unstable environment east of the Lesser Antilles. These conflicting factors lead to a forecast of little change in the winds of Isaac until the cyclone moves into the eastern Caribbean. I should mention that the short-term trend on satellite makes me think the intensity forecast is too high over the next 36 hours. Yet, given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, this is the course of least regret. We can see if this trend becomes more consistent before making substantial changes on the next advisory. Model guidance is very divergent in the long range for Isaac, with the regional hurricane and statistical models showing intensification, while the global models show continued weakening. The global models have done fairly well with this cyclone, so the forecast will lean more heavily on their weaker solutions.

The latest initial motion estimate is 270/15, a little faster than before. Isaac is forecast to move westward at roughly the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Model guidance is tightly clustered on this cycle, with the UKMET now coming into good agreement with the rest of the guidance. The only change to this forecast is to accelerate Isaac roughly 1 kt over the course of the prediction, which over the course of a 5-day forecast results in a sizable westward shift. This forecast could still be too slow, since the GFS and ECMWF are a bit ahead of the new NHC track prediction.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials, and warnings will likely be issued this evening.

2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days.

FORECAST POSITIONS AND MAX WINDS

INIT  11/2100Z 14.6N  51.3W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 12H  12/0600Z 14.6N  53.4W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 24H  12/1800Z 14.8N  56.2W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 36H  13/0600Z 15.0N  59.2W   60 KT  70 MPH - Tropical Storm (ESE Rosalie, Dominica)
 48H  13/1800Z 15.3N  62.3W   55 KT  65 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 72H  14/1800Z 15.5N  68.5W   50 KT  60 MPH - Tropical Storm (ESE La Romana,  Dominican Republic)
 96H  15/1800Z 15.5N  73.5W   45 KT  50 MPH - Tropical Storm (S Les Cayes, Haiti)
120H  16/1800Z 15.5N  77.5W   45 KT  50 MPH - Tropical Storm (WSW Hayes, Jamaica)

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

Microwave and visible satellite data show that the center of Isaac is partially exposed on the western edge of the central dense overcast. Perhaps Isaac is a little weaker than earlier, but a blend of the latest subjective and objective estimates still gives an uncertain initial wind speed of about 60 kt. We should have a better idea of the maximum winds by tomorrow morning when a NOAA Hurricane Hunter enters the storm, or if we get lucky with a scatterometer pass this evening.

Isaac is barely holding on in the face of increasing shear, which is much more evident in the 300-500 mb layer than a conventional analysis at 200 mb. However, convection could redevelop near the center while the storm moves over warming SSTs into a more moist and unstable environment east of the Lesser Antilles. These conflicting factors lead to a forecast of little change in the winds of Isaac until the cyclone moves into the eastern Caribbean. I should mention that the short-term trend on satellite makes me think the intensity forecast is too high over the next 36 hours. Yet, given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, this is the course of least regret. We can see if this trend becomes more consistent before making substantial changes on the next advisory. Model guidance is very divergent in the long range for Isaac, with the regional hurricane and statistical models showing intensification, while the global models show continued weakening. The global models have done fairly well with this cyclone, so the forecast will lean more heavily on their weaker solutions.

The latest initial motion estimate is 270/15, a little faster than before. Isaac is forecast to move westward at roughly the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Model guidance is tightly clustered on this cycle, with the UKMET now coming into good agreement with the rest of the guidance. The only change to this forecast is to accelerate Isaac roughly 1 kt over the course of the prediction, which over the course of a 5-day forecast results in a sizable westward shift. This forecast could still be too slow, since the GFS and ECMWF are a bit ahead of the new NHC track prediction.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials, and warnings will likely be issued this evening.

2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days.

FORECAST POSITIONS AND MAX WINDS

INIT  11/2100Z 14.6N  51.3W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 12H  12/0600Z 14.6N  53.4W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 24H  12/1800Z 14.8N  56.2W   60 KT  70 MPH - Tropical Storm (ESE Sainte-Anne, Martinique)
 36H  13/0600Z 15.0N  59.2W   60 KT  70 MPH - Tropical Storm (ESE Rosalie, Dominica)
 48H  13/1800Z 15.3N  62.3W   55 KT  65 MPH - Tropical Storm (WSW Portsmouth, Dominica)
 72H  14/1800Z 15.5N  68.5W   50 KT  60 MPH - Tropical Storm (S Saona Island, Dominican Republic)
 96H  15/1800Z 15.5N  73.5W   45 KT  50 MPH - Tropical Storm (S Les Cayes, Haiti)
120H  16/1800Z 15.5N  77.5W   45 KT  50 MPH - Tropical Storm (Se Treasure Beach, Jamaica)

NWS National Hurricane Center Miami FL 200 PM AST Tue Sep 11 2018

DISCUSSION AND OUTLOOK

At 200 PM AST (1800 UTC), the center of Tropical Storm Isaac was located with high-resolution GOES-16 visible satellite data near latitude 14.6 North, longitude 50.4 West. Isaac is moving toward the west near 16 mph (26 km/h), and this motion is expected to continue for the next few days. On the forecast track Isaac is anticipated to move near or over the central Lesser Antilles on Thursday and move into the eastern Caribbean Sea Thursday night.

Maximum sustained winds remain near 70 mph (110 km/h) with higher gusts. Isaac is expected to be near hurricane strength when it moves through the central Lesser Antilles, with some weakening forecast afterward on Friday.Tropical Storm Isaac Tropical Force Wind Speed 1400 Hours September 11 2018

Tropical-storm-force winds extend outward up to 105 miles (165 km) from the center.

The estimated minimum central pressure is 996 mb (29.42 inches).

HAZARDS AFFECTING LAND

RAINFALL: Isaac is expected to produce total rainfall accumulations of 3 to 5 inches with isolated amounts near 10 inches across the southern Leeward Islands late this week, with 1 to 2 inches anticipated across portions of the Windward Islands.

STORM SURGE: A storm surge of 2 to 4 feet above normal tide levels is possible near and to the north of where the center moves through the Lesser Antilles. Near the coast, the surge will be accompanied by large and destructive waves.

WIND: Hurricane conditions are possible within the hurricane watch area by Thursday morning, with tropical storm conditions possible early Thursday in both the hurricane and tropical storm watch areas.

SURF: Swells generated by Isaac will begin to affect portions of the Lesser Antilles on Wednesday afternoon. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

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

One-minute visible satellite data from GOES-16 indicate that the center of Isaac remains on the westward edge of the central dense overcast. The initial wind speed is kept at 60 kt, which is a compromise between the lower subjective estimates and higher ADTs and microwave estimates. This is a more uncertain estimate than normal due to the spread of the intensity fixes. The initial wind and 12 ft seas radii have been expanded out to the north based on TAFB estimates and microwave data.

The intensity forecast continues to be tricky. Westerly shear should increase over the next couple of days, which would normally cause some weakening. However this effect could be tempered by increasing SSTs, mid-level moisture and instability in the path of the storm. Given our limited skill in forecasting intensity changes in marginal environments, plus the 5 or 10 kt of initial wind speed uncertainty, the official forecast will show no significant change in intensity for the next couple of days. Because of these uncertainties, a Hurricane Watch has been issued for many of the Leeward Islands, even though Isaac is not explicitly forecast to be a hurricane. Some weakening is indicated at long range after considering the global model trends in intensity, which have done fairly well for Isaac.Tropical Storm Isaac Tropical Force Wind Speed 1100 Hours September 11 2018

The latest initial motion estimate is 270/14. Isaac is forecast to move westward at about the same speed for the next few days due to a mid-level ridge over the central Atlantic Ocean. Other than the UKMET model, there is not much spread in the guidance, except with the forward speed. Given the strength of the ridge, the cyclone isn’t likely to slow down much, so the official forecast is basically along the previous forecast track but faster. The uncertainty increases at long range, and is somewhat dependent on the intensity of Isaac, but faster seems to be way to go given the weakening trend anticipated at that time.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles on Thursday, and hurricane and tropical storm watches have been issued for several islands. Interests in those areas should follow any advice given by their local officials.

2. Interests elsewhere in the Lesser Antilles should continue to monitor Isaac during the next couple of days, as additional watches could be needed for other islands.

FORECAST POSITIONS AND MAX WINDS

INIT  11/1500Z 14.6N  49.7W   60 KT  70 MPH - Tropical Storm (ENE Sainte-Anne, Martinique)
 12H  12/0000Z 14.6N  51.8W   60 KT  70 MPH - Tropical Storm (ENE Sainte-Anne, Martinique)
 24H  12/1200Z 14.7N  54.4W   65 KT  75 MPH - Tropical Storm (ENE Sainte-Anne, Martinique)
 36H  13/0000Z 14.9N  57.1W   60 KT  70 MPH - Tropical Storm (ESE Rosalie, Dominica)
 48H  13/1200Z 15.1N  59.9W   60 KT  70 MPH - Tropical Storm (ESE Rosalie, Dominica)
 72H  14/1200Z 15.5N  65.5W   55 KT  65 MPH - Tropical Storm (ESE Portsmouth,Dominica)
 96H  15/1200Z 15.5N  71.0W   50 KT  60 MPH - Tropical Storm (WSW Juancho, Dominican Republic)
120H  16/1200Z 15.5N  75.0W   50 KT  60 MPH - Tropical Storm (ESE Morant Bay,Jamaica)

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

Hurricane Isaac’s cloud pattern is currently a bit ragged-looking, with an irregular CDO and limited convective banding. Dvorak intensity estimates from both TAFB and SAB are 65 kt, and that value will be used for the advisory. There is significant uncertainty in the intensity forecast for this system. Vertical wind shear over the hurricane is forecast to be weak to moderate during the next day or so, which should allow Isaac to strengthen somewhat. By 48 hours, however, the global models depict significantly stronger northwesterly shear, associated with an upper-level trough over the central Atlantic, affecting the tropical cyclone. Most of the guidance does not show Isaac strengthening over the Caribbean. The HWRF model is an outlier, strengthening Isaac into a major hurricane later in the period, and it is the main contributor to high consensus forecasts at days 3-5. The NHC intensity forecast is above the model consensus through 72 hours and below it by the end of the period.

Isaac continues to move westward, or 275/12 kt. A subtropical ridge should be maintained to the north of the tropical cyclone for the next several days, so a continued westward track seems likely. The U. K. Met. Office model is an outlier and shows Isaac turning northwestward and northward to the northeast of the Leeward Islands. The official track forecast, like the previous one, follows the other reliable global models and is also close to the HFIP corrected consensus guidance.Hurricane Isaac Tropical Force Winds 1100 Hours September 10 2018

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is greater than normal. Although Isaac is forecast to begin weakening when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands.
2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.7N  43.9W   65 KT  75 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 12H  11/0000Z 14.9N  45.9W   70 KT  80 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 24H  11/1200Z 15.0N  48.7W   75 KT  85 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 36H  12/0000Z 15.0N  51.4W   75 KT  85 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 48H  12/1200Z 15.1N  54.0W   75 KT  85 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 72H  13/1200Z 15.4N  59.4W   70 KT  80 MPH - Category 1 (ENE Rosalie, Dominica)
 96H  14/1200Z 15.5N  64.5W   60 KT  70 MPH - Tropical Storm (SE Frederiksted, USVI)
120H  15/1200Z 15.7N  70.0W   50 KT  60 MPH - Tropical Storm (WSW Santo Domingo, Dominican Republic)

NWS National Hurricane Center Miami FL 500 AM AST Mon Sep 10 2018

Corrected intensity forecast in table below. Isaac  has generally changed little during the past several hours. The small hurricane continues to have a central dense overcast feature with some fragmented bands to the south of the center. The Dvorak classifications remain 4.0/65 kt from both TAFB and SAB, and therefore, the initial intensity is held at 65 kt.

Isaac is moving westward at 11 kt. The models remain in poor agreement this cycle as they are split into two camps. The UKMET, HWRF, and a few other models show Isaac recurving near or to the east of the Lesser Antilles toward a developing trough over the central Atlantic. Conversely, the GFS and ECMWF have been very consistent in showing a due westward motion across the Lesser Antilles and into the Caribbean as the cyclone moves on the south side of a strengthening mid-level ridge. The NHC track forecast continues to favor the usually more reliable GFS and ECMWF models, and this forecast is near a blend of those aids. It should be noted, however, that confidence in this track forecast is low given the model spread.Hurricane Isaac Earliest Arrival Time Tropical Force Winds 0500 Hours September 10 2018

The intensity forecast is also quite uncertain. One of the complex factors is Isaac’s compact size as small hurricanes like this one are more prone to quick intensity changes, both up and down. In addition, the intensity guidance is in poor agreement as solutions range from steady weakening from this point onward to the HWRF that shows Isaac becoming a major hurricane. Given that the wind shear is expected to remain less than 10 kt for another day or two, strengthening is forecast during that time period. After that, however, an increase in shear first from a trough over the central Atlantic and later from the outflow from Florence should result in weakening when the system moves across the Lesser Antilles and into the Caribbean Sea. The NHC intensity forecast is not too different from the previous one and lies above the model consensus in the short term, but below it at the later forecast times. This forecast is in best agreement with the latest LGEM guidance.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is higher than normal. Although Isaac is forecast to begin weakening when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0900Z 14.7N  42.7W   65 KT  75 MPH - Category 1 (WSW São Filipe, Cape Verde)
 12H  10/1800Z 14.8N  44.6W   70 KT  80 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 24H  11/0600Z 15.0N  47.4W   75 KT  85 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 36H  11/1800Z 15.0N  50.3W   80 KT  90 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 48H  12/0600Z 15.0N  53.0W   80 KT  90 MPH - Category 1 (ENE Sainte-Anne, Martinique)
 72H  13/0600Z 15.3N  58.0W   70 KT  80 MPH - Category 1 (E Rosalie, Dominica)
 96H  14/0600Z 15.5N  64.0W   60 KT  70 MPH - Tropical Storm (W Portsmouth, Dominica)
120H  15/0600Z 15.6N  69.2W   50 KT  60 MPH - Tropical Storm (ESE Santo Domingo, Dominican Republic)

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

Isaac’s  IR satellite presentation has generally improved since the last advisory. While there is no evidence of an eye in conventional satellite imagery, the most recent microwave imagery around 2100 UTC indicated the presence of a pinhole eye. Dvorak intensity estimates are T4.0 from both TAFB and SAB, and on this basis Isaac has been upgraded to a hurricane, the 5th of the 2018 Atlantic season and 3rd in the past few days.

I must stress that all aspects of the forecast are highly uncertain at this time. The dynamical track model spread is nearly 1000 miles at day 5, while the intensity guidance spread is nearly 100 kt. Although the NHC track and intensity forecasts have not been changed very much, this should not be interpreted as increasing confidence in the forecast.

With regard to track, there is a clear split in the model guidance regarding the speed of Isaac. The GFS and ECMWF models forecast that a mid-level ridge to the north will accelerate the hurricane westward for the next 36 h, with a westward motion continuing through day 5 as the ridge is reinforced in the wake of Florence. On the other hand, the UKMET and HWRF models show a weakness developing in the ridge which causes Isaac to move much slower, and eventually turn northward. The NHC forecast has been favoring the former solution, and continues to do so with this advisory since I see no reason to make a drastic change to the forecast at this point.

Hurricane Isaac Tropical Force Winds 2300 Hours September 9 2018
Hurricane Isaac Tropical Force Winds 2300 Hours September 9 2018

Isaac is a very small hurricane, which makes the intensity forecast particularly tricky since small storms are susceptible to rapid changes of intensity, up and down. Since the wind shear is expected to remain low, and Isaac is forecast to remain over fairly warm SSTs for the next 48 h, the current forecast of further intensification seems reasonable, and the NHC forecast is still at the top end of the guidance envelope. From 72 h onward, there could be a sudden, and substantial increase in wind shear associated first with an upper-level trough to the north, and later with the outflow of Hurricane Florence. Assuming this shear affects Isaac as expected, the tiny hurricane would likely weaken quickly, perhaps even faster than currently shown in the forecast. By the end of the forecast period, the NHC forecast remains a fair amount below the intensity consensus, closer to the weakening solutions of the GFS and ECMWF.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is higher than normal. Although Isaac is forecast to begin weakening by Tuesday when it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0300Z 14.5N  41.6W   65 KT  75 MPH - Category 1 (WSW São Filipe, Cape Verde)
 24H  11/0000Z 14.9N  46.3W   80 KT  90 MPH - Category 1 (NE Sainte-Anne, Martinique)
 36H  11/1200Z 15.0N  49.3W   85 KT 100 MPH - Category 2 (NE Sainte-Anne, Martinique)
 48H  12/0000Z 15.0N  52.0W   85 KT 100 MPH - Category 2 (NE Sainte-Anne, Martinique)
 72H  13/0000Z 15.2N  57.3W   75 KT  85 MPH - Category 1 (E Rosalie,Dominica)
 96H  14/0000Z 15.5N  63.5W   60 KT  70 MPH - Tropical Storm (W Portsmouth, Dominica)
120H  15/0000Z 15.5N  70.0W   50 KT  60 MPH - Tropical Storm (S Santo Domingo, Dominican Republic)

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

Isaac is a tiny tropical cyclone, but even still, recent SSMIS data showed that it has developed a mid-level microwave eye feature. At the time of the pass (1800 UTC), the convection was a little thin on the southern side, but geostationary satellite images suggest that it has filled in since that time. Dvorak intensity estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum winds are estimated to be 60 kt.

The intensity forecast philosophy has not changed since this morning. Isaac is strengthening in an environment of low shear and over warm sea surface temperatures, and for the next 36-48 hours, the NHC intensity forecast is above the bulk of the intensity models. The main reason for this is that Isaac’s tiny size could allow the intensity to increase quickly within the favorable environment. However, the cyclone’s tiny size will likely also be to its detriment after 48 hours when northwesterly vertical shear is expected to develop and increase to 20-30 kt. The shear will probably easily decouple the small system, causing the intensity to decrease much faster than suggested by most of the intensity models. For that reason, the official NHC intensity forecast is below the intensity consensus on days 3 through 5. It should be noted that both the GFS and ECMWF models show a weakening cyclone moving into the eastern Caribbean Sea by days 4 and 5, with the GFS even making the system an open wave by the end of the forecast period. These global model solutions lend credence to the belief that it may be difficult for Isaac to maintain hurricane status while it approaches the Lesser Antilles.Tropical Storm Isaac Tropical Storm Force Winds 1700 Hours September 9 2018

Isaac is accelerating toward the west with an initial motion of 275/10 kt. Ridging to the north is expected to keep the cyclone on a westward trajectory for the entire forecast period, with acceleration continuing for the next 36 hours. The UKMET remains the biggest outlier, showing Isaac turning northwestward and northward into the central Atlantic after 48 hours. That still appears to be unlikely at this time, and the NHC track forecast is still close to the center of the guidance envelope. Only a slight northward shift was made to the new forecast based on the latest model solutions.

Key Messages:

1. Isaac is expected to become a hurricane tonight or on Monday while it moves westward across the tropical Atlantic Ocean.

2. While Isaac is forecast to begin weakening by Tuesday while it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands, and the uncertainty in the intensity forecast is higher than usual. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  09/2100Z 14.5N  40.3W   60 KT  70 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  10/0600Z 14.6N  42.2W   70 KT  80 MPH - Category 1 (NE Sainte-Anne, Martinique)
 24H  10/1800Z 14.8N  45.0W   80 KT  90 MPH - Category 1 (NE Sainte-Anne, Martinique)
 36H  11/0600Z 14.9N  48.0W   85 KT 100 MPH - Category 2 (NE Sainte-Anne, Martinique)
 48H  11/1800Z 14.9N  50.9W   85 KT 100 MPH - Category 2 (SE Sainte-Anne, Martinique)
 72H  12/1800Z 15.0N  56.5W   70 KT  80 MPH - Category 1 (SE Rosalie, Dominica)
 96H  13/1800Z 15.5N  63.0W   60 KT  70 MPH - Category 1 (NW Portsmouth, Dominica)
120H  14/1800Z 15.5N  69.5W   50 KT  60 MPH - Tropical Storm (SE Santo Domingo Dominican Republic)

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

Tropical Storm Isaac – WindSat microwave data from several hours ago show that Isaac has developed a well-defined low-level ring in the 37-GHz channel, and the center is embedded in the middle of a small central dense overcast. TAFB and SAB Dvorak estimates have increased to T3.5 given Isaac’s improved structure, so the initial intensity is raised to 55 kt.

Isaac’s future intensity is the most challenging part of the forecast. Since sea surface temperatures will be warm (between 27-28 degrees Celsius) and vertical shear will be almost non-existent (less than 10 kt) for the next 36-48 hours, the tiny storm will be prone to a possibly significant increase in intensity during the next couple of days. For that period, the NHC intensity forecast is above the guidance, and above the previous forecast, due to relatively high numbers from the various rapid intensification indices. However, in 2-3 days, the global models are showing northwesterly shear steadily increasing and potentially reaching a maximum of around 30 kt by Thursday.

Tropical Storm Isaac Track Earliest Arrival Times 1100 Hours September 9 2018
Tropical Storm Isaac Track Earliest Arrival Times 1100 Hours September 9 2018

The big question is the timing of this shear and how much it will affect Isaac’s intensity before the cyclone reaches the Lesser Antilles. The HWRF, which brings Isaac to major hurricane intensity despite the forecast shear, appears to be an outlier, and is therefore pulling up the ICON intensity consensus and HCCA. As a result, the NHC intensity forecast is below ICON on days 4 and 5. This new forecast shows a more peaked maximum intensity, with Isaac possibly increasing and then decreasing in strength at faster rates than shown here.

While the microwave data indicate that Isaac’s center is slightly south of previous estimates, the current motion estimate is 270/8 kt. A low- to mid-level ridge is expected to move westward in tandem with Isaac for much of the forecast period, causing the cyclone to accelerate toward the west, reaching a forward speed of around 15 kt by 36 hours, and then holding that speed through day 5. With the exception of the UKMET and HWRF models, the guidance envelope is tightly packed from north to south, and the speed differences noted over the past few days have decreased. The updated NHC track forecast is just a little south of the previous one to account for the adjusted initial position, and lies slightly south of the multi-model consensus in the direction of HCCA and the Florida State Superensemble.

FORECAST POSITIONS AND MAX WINDS

INIT  09/1500Z 14.3N  39.1W   55 KT  65 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  10/0000Z 14.4N  40.8W   65 KT  75 MPH - Category 1 (E Sainte-Anne, Martinique)
 24H  10/1200Z 14.4N  43.3W   75 KT  85 MPH - Category 1 (E Sainte-Anne, Martinique)
 36H  11/0000Z 14.4N  46.3W   85 KT 100 MPH - Category 2 (E Sainte-Anne, Martinique)
 48H  11/1200Z 14.3N  49.1W   85 KT 100 MPH - Category 2 (E Sainte-Anne, Martinique)
 72H  12/1200Z 14.2N  54.8W   80 KT  90 MPH - Category 1 (ESE Sainte-Anne, Martinique)
 96H  13/1200Z 14.5N  60.5W   70 KT  80 MPH - Category 1 (SE Le Vauclin, Martinique)
120H  14/1200Z 15.0N  66.5W   60 KT  70 MPH - Category 1 (S Ponce, Puertyp Rico)

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

The cloud pattern of Isaac  has changed significantly during the past several hours. The storm no longer has a sheared appearance, and instead microwave images reveal a well-defined curved band that wraps a little more than halfway around the center. Despite the change in the storm’s structure, the Dvorak classifications are unchanged at 3.0/45 kt. Based on that data, the initial intensity is held at 45 kt for this advisory.

Now that the shear has lessened, Isaac should steadily strengthen during the next few days as the storm remains over warm waters and in a low wind shear environment. Beyond a few days, however, the SHIPS model shows a notable increase in shear, in part due to the outflow from Florence, which should end the strengthening trend and induce some weakening. There was no significant change in the intensity guidance this cycle, so the official forecast is largely an update of the previous one. This prediction lies fairly close to the IVCN and HCCA models.

Hurricane Issac Wind Speeds Sept 9 to Sept 14 2018
Hurricane Issac Wind Speeds Sept 9 to Sept 14 2018

Isaac remains on the forecast track and the initial motion is the same as before, 270/7. A strengthening subtropical ridge to the north of the system should cause Isaac to move westward at an increasing forward speed during the next several days. Although there is some spread in the models, the usually more reliable GFS and ECMWF models have been consistent in showing a due westward motion through the forecast period. The NHC track forecast leans toward those models, and it is fairly similar to the previous prediction. Based on the current forecast, Isaac will be near the Lesser Antilles in 4 to 5 days and interests there should monitor the progress of this system.

FORECAST POSITIONS AND MAX WINDS

INIT  09/0900Z 14.5N  38.1W   45 KT  50 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  09/1800Z 14.6N  39.6W   55 KT  65 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 24H  10/0600Z 14.7N  41.9W   60 KT  70 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 36H  10/1800Z 14.7N  44.6W   70 KT  80 MPH - Category 1 (E Sainte-Anne, Martinique)
 48H  11/0600Z 14.7N  47.5W   80 KT  90 MPH - Category 1 (E Sainte-Anne, Martinique)
 72H  12/0600Z 14.7N  53.1W   85 KT 100 MPH - Category 2 (E Sainte-Anne, Martinique)
 96H  13/0600Z 14.9N  58.9W   80 KT  90 MPH - Category 1 (E Rosalie,Dominica)
120H  14/0600Z 15.3N  64.5W   70 KT  80 MPH - Category 1 (WNW Roseau,Dominica)

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

The structure of the cyclone , while still a shear pattern, is improving, with the low-level center now near the convective overcast and improved outflow in the eastern semicircle. Satellite intensity estimates have increased to the 30-45 kt range, with the CIMSS satellite consensus technique now suggesting winds near 35 kt. Based on these estimates, the initial intensity is increased to 35 kt and the depression is upgraded to Tropical Storm Isaac.

Isaac is now moving slowly westward, with the initial motion 275/6. The subtropical ridge to the north of Isaac is forecast to steer the system westward with an increasing forward speed for at least the next 2-3 days. After that time, there is some uncertainty in the track forecast due to the formation of a mid- to upper-level trough over the central Atlantic, with some of the track guidance showing a decrease in forward speed and a more northerly motion near the end of the forecast period. The GFS and ECMWF models show a continued westerly motion through that time, and the official forecast follows that scenario, although with a slower forward speed than in the previous forecast. The new forecast is again similar to the earlier forecast and lies near the various consensus models.Hurricane Issac Wind Speeds Sept 8 to Sept 13 2018

Conditions appear favorable for gradual strengthening during the next 72 h as Isaac moves westward across the warm waters of the tropical Atlantic. In best agreement with the SHIPS model for the next three days, the new intensity forecast is raised from the previous forecast during this time. From 72-120 h, the system is expected to encounter westerly wind shear, which is expected to cause some weakening.

FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 14.5N  36.6W   35 KT  40 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  09/0600Z 14.6N  37.7W   40 KT  45 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 24H  09/1800Z 14.7N  39.5W   50 KT  60 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 36H  10/0600Z 14.7N  42.0W   60 KT  70 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 48H  10/1800Z 14.7N  44.6W   65 KT  75 MPH - Category 1 (E Sainte-Anne, Martinique)
 72H  11/1800Z 14.5N  50.0W   75 KT  85 MPH - Category 1 (E Sainte-Anne, Martinique)
 96H  12/1800Z 14.5N  55.5W   75 KT  85 MPH - Category 1 (E Sainte-Anne, Martinique)
120H  13/1800Z 15.0N  61.0W   70 KT  80 MPH - Category 1 (SE Rosalie, Dominica)

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

The structure of Tropical Depression Nine  is improving as the ongoing shear diminishes, with the convection getting closer to the low-level center. However, recent scatterometer data indicates that the system has not yet become a tropical storm, and the initial intensity remains 30 kt.

The cyclone is still moving fairly slowly, with the initial motion now 300/5. The subtropical ridge to the north of the cyclone is forecast to steer the system westward with an increasing forward speed for at least the next 2-3 days. After that time, there is some uncertainty in the track forecast due to the formation of a mid- to upper-level trough over the central Atlantic. The Canadian and UKMET models forecast the cyclone to slow down and turn northwestward near the end of the forecast period due to the trough, while the ECMWF and GFS show a continued westward motion. The track forecast follows the ECMWF/GFS scenario and shows a faster westward motion than the previous forecast. The new forecast also lies near the various consensus models.

With the shear diminishing, conditions are becoming more favorable for the cyclone to strengthen during the next 72 h. However, there are still concerns about dry air entraining into the system. The new intensity forecast is similar to the previous forecast with the cyclone becoming a tropical storm in about 12 h and a hurricane in about 72 h. After 72 h, the system is expected to encounter another round of shear, which is expected to cause some weakening.

FORECAST POSITIONS AND MAX WINDS

INIT  08/1500Z 14.6N  36.0W   30 KT  35 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  09/0000Z 14.7N  36.9W   35 KT  40 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 24H  09/1200Z 14.8N  38.5W   40 KT  45 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 36H  10/0000Z 14.8N  40.8W   50 KT  60 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 48H  10/1200Z 14.8N  43.5W   60 KT  70 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 72H  11/1200Z 14.5N  49.0W   75 KT  85 MPH - Category 1 (E Sainte-Anne, Martinique)
 96H  12/1200Z 14.5N  55.0W   80 KT  90 MPH - Category 1 (E Sainte-Anne, Martinique)
120H  13/1200Z 14.5N  60.0W   75 KT  85 MPH - Category 1 (E Sainte-Anne, Martinique)

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

The depression continues to be affected by moderate to strong east to southeasterly vertical wind shear, which has resulted in the bulk of the deep convection being displaced to the west of the low-level center. The initial intensity has been held at 30 kt based on Dvorak satellite intensity estimate of T2.0/30 kt from both TAFB and SAB.

The depression has been meandering in the same general area for the past 12-18 h due to being caught in a weakness in the subtropical ridge. However, recent microwave satellite data suggest that the cyclone is now moving slowly northwestward. A turn toward the west is expected to occur later today as a shortwave trough to the north that has weakened the ridge lifts out, allowing the ridge to fill back in and build westward to the north of the Lesser Antilles. The latest NHC model guidance suite remains tightly packed, and the official forecast track lies down the middle of the envelope near a blend of the consensus aids TCVA, HCCA, FSSE.

Although the wind shear is forecast to gradually weaken and become less than 5 kt by 36-48 h, which would favor rapid strengthening, the airmass that the cyclone is embedded is fairly dry. Since mid-level humidity values are forecast to decrease from the current 60-percent range down to around 50 percent by 48 hours and beyond, only modest strengthening is expected during the next 4 days. By day 5, vertical wind shear from the southwest is forecast to increase to 20-25 kt, serving to cap the intensification process. The NHC intensity forecast is based on a blend of the IVCN and the HCCA consensus models, and continues to remain below the stronger HWRF model at days 4 and 5, which makes the cyclone a major hurricane in 78 h.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 14.3N  35.4W   30 KT  35 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  08/1800Z 14.4N  36.1W   35 KT  40 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 24H  09/0600Z 14.5N  37.5W   40 KT  45 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 36H  09/1800Z 14.7N  39.5W   45 KT  50 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 48H  10/0600Z 14.6N  42.0W   55 KT  65 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 72H  11/0600Z 14.5N  47.3W   75 KT  85 MPH - Category 1 (ENE Martinique)
 96H  12/0600Z 14.6N  52.7W   80 KT  90 MPH - Category 1 (ENE Martinique)
120H  13/0600Z 14.9N  57.2W   80 KT  90 MPH - Category 1 (ENE Martinique)

NWS National Hurricane Center Miami FL 500 PM AST Fri Sep 07 2018

Corrected bin number from 2 to 4  – Satellite images indicate that the low pressure system  over the eastern tropical Atlantic has developed a well-defined center and sufficient deep convection to be classified a tropical depression, the ninth one of the 2018 Atlantic hurricane season and second one of the day. The initial intensity is set to 30 kt based on a 2.0 Dvorak classification from TAFB and earlier ASCAT data. The depression has not moved much today, and it has even wobbled a bit to the east recently. This slow motion is a result of very weak steering with broad troughing to the north and Tropical Depression Eight to its east. Little motion is expected through tonight, but a westward motion with a gradual increase in forward speed is forecast thereafter as a subtropical ridge builds to the north of the cyclone. The NHC track forecast lies closest to the HCCA guidance and takes the system toward the Lesser Antilles by the end of the forecast period.

The depression is experiencing some influences of easterly shear, which is why the center is currently on the eastern edge of the convection. Little change in intensity is expected during the next 12 to 24 hours since the cyclone is still in the developing stage and because moderate shear is expected to continue. However, the shear is expected to decrease to less than 10 kt in about 24 hours. These more favorable winds aloft combined with relatively warm waters and a fairly moist airmass should allow for steady strengthening beginning later in the weekend. There is a large spread in the intensity guidance, with the HWRF showing the system becoming a major hurricane and HMON showing almost no strengthening. The NHC intensity forecast lies a little below consensus models since it appears that it will take some time for the system to strengthen.

FORECAST POSITIONS AND MAX WINDS

INIT  07/2100Z 13.6N  34.9W   30 KT  35 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 12H  08/0600Z 13.9N  35.2W   30 KT  35 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 24H  08/1800Z 14.0N  36.2W   30 KT  35 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 36H  09/0600Z 14.0N  37.6W   40 KT  45 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 48H  09/1800Z 14.0N  39.3W   50 KT  60 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 72H  10/1800Z 14.0N  44.0W   60 KT  70 MPH - Tropical Storm (WSW São Filipe, Cape Verde)
 96H  11/1800Z 14.1N  48.9W   70 KT  80 MPH - Category 1 (E Saint Lucia)
120H  12/1800Z 14.5N  54.5W   75 KT  85 MPH - Category 1 (E Martinique)

NWS National Hurricane Center Miami FL 800 AM EDT Fri Sep 7 2018 

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Hurricane Florence, located over the central Atlantic Ocean. The Weather Prediction Center is issuing advisories on Tropical Depression Gordon, located over Arkansas.

1. Satellite images indicate that the circulation of a low pressure system located about 650 miles west of the Cabo Verde Islands is gradually becoming better defined. In addition, the associated showers and thunderstorms remain fairly concentrated near the center. This system is expected to become a tropical depression later today or tonight while it moves slowly westward across the tropical Atlantic Ocean. * Formation chance through 48 hours…high…90 percent. * Formation chance through 5 days…high...90 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Thu Sep 6 2018 

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Hurricane Florence, located over the central Atlantic Ocean. The Weather Prediction Center is issuing advisories on Tropical Depression Gordon, located over Arkansas.

2. A tropical wave and an accompanying low pressure system near the west coast of Africa is producing a large area of thunderstorms. Environmental conditions appear conducive for development, and a tropical depression or tropical storm is likely to form over the weekend while the wave moves westward or west-northwestward over the far eastern tropical Atlantic Ocean. Interests in the Cabo Verde Islands should monitor the progress of this system, and potential tropical cyclone advisories could be issued on Friday. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…90 percent.

NWS National Hurricane Center Miami FL 515 PM EDT Thu Sep 6 2018 

Special outlook issued to update probabilities for the far eastern Atlantic system. For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Hurricane Florence, located over the central subtropical Atlantic Ocean. The Weather Prediction Center is issuing advisories on Tropical Depression Gordon, located over Arkansas.

2. Updated: A tropical wave with an accompanying low pressure area is forecast to move off the west coast of Africa on Friday. This system is already showing signs of organization, and a tropical depression is likely to form over the weekend or early next week while the wave moves westward or west-northwestward over the far eastern tropical Atlantic Ocean. Interests in the Cabo Verde Islands should monitor the progress of this system. * Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…high…80 percent.

Tropical Storm Isaac Forming – NWS National Hurricane Center Miami FL 200 PM EDT Thu Sep 6 2018

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

The National Hurricane Center is issuing advisories on Hurricane Florence, located over the central subtropical Atlantic Ocean. The Weather Prediction Center is issuing advisories on Tropical Depression Gordon, located over Arkansas.

A tropical wave is forecast to move off the west coast of Africa on Friday. Development of this system is anticipated after that time, and a tropical depression could form over the weekend or early next week while the wave moves westward or west-northwestward over the far eastern tropical Atlantic Ocean. Interests in the Cabo Verde Islands should monitor the progress of this system during the next several days. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…medium…60 percent.

If this storm system is named, it will be Tropical Storm Isaac

NWS National Hurricane Center Miami FL 800 AM EDT Thu Sep 6 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Hurricane Florence, located over the central subtropical Atlantic Ocean. The Weather Prediction Center is issuing advisories on Tropical Depression Gordon, located near the Mississippi-Arkansas border.

2. A tropical wave is forecast to move off the west coast of Africa on Friday. Some development of this system is anticipated after that time, and a tropical depression could form over the weekend or early next week while the wave moves westward or west-northwestward over the far eastern tropical Atlantic Ocean. Interests in the Cabo Verde Islands should monitor the progress of this system during the next several days. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…medium…50 percent.

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Video: UPDATE BY PM LEONA ROMEO MARLIN ON TROPICAL STORM ISAAC

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