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Tropical Storm Isaac Forecast Discussion Number 24

2018-09-13 16:41:18| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 13 2018 764 WTNT44 KNHC 131441 TCDAT4 Tropical Storm Isaac Discussion Number 24 NWS National Hurricane Center Miami FL AL092018 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. 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 12H 14/0000Z 14.9N 64.2W 35 KT 40 MPH 24H 14/1200Z 14.9N 67.0W 35 KT 40 MPH 36H 15/0000Z 14.9N 69.8W 35 KT 40 MPH 48H 15/1200Z 15.0N 72.3W 35 KT 40 MPH 72H 16/1200Z 15.5N 76.5W 35 KT 40 MPH 96H 17/1200Z 17.0N 80.5W 30 KT 35 MPH 120H 18/1200Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Helene Forecast Discussion Number 25

2018-09-13 16:39:21| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 13 2018 266 WTNT43 KNHC 131439 TCDAT3 Tropical Storm Helene Discussion Number 25 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Thu Sep 13 2018 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 12H 14/0000Z 27.2N 37.1W 55 KT 65 MPH 24H 14/1200Z 30.8N 36.6W 50 KT 60 MPH 36H 15/0000Z 34.2N 35.5W 50 KT 60 MPH 48H 15/1200Z 37.2N 33.3W 50 KT 60 MPH 72H 16/1200Z 40.5N 26.0W 50 KT 60 MPH 96H 17/1200Z 44.0N 18.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 18/1200Z 51.5N 9.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Landsea

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Subtropical Storm Joyce Forecast Discussion Number 4

2018-09-13 16:37:47| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 13 2018 594 WTNT45 KNHC 131437 TCDAT5 Subtropical Storm Joyce Discussion Number 4 NWS National Hurricane Center Miami FL AL102018 1100 AM AST Thu Sep 13 2018 Joyce's satellite presentation is less than impressive this morning, with the low-level center exposed to the northwest of a small patch of deep convection. The initial intensity is set to 35 kt based on the latest subtropical classification of ST2.5 from TAFB a 1208Z ASCAT-B overpass that showed 30-35 kt winds north and northeast of the center. Cyclone phase space diagrams still show Joyce with a shallow-to-moderate warm core, and the system remains a subtropical storm for now. Little change in intensity is expected during the next few days, as Joyce will continue to be affected by strong westerly-to- southwesterly vertical wind shear. The new NHC forecast is a little below the intensity consensus given the weakening trend seen in the global models. Dissipation is shown at 96 hours, but it wouldn't be surprising if Joyce becomes a post-tropical cyclone before that. The initial motion estimate is 245/05. Joyce is currently situated a little to the west of an upper-level low, which is currently steering the system southwestward and southward, and that should continue for the next day or so. Then, Joyce should begin to accelerate northeastward ahead of an approaching mid-level trough. The new NHC track forecast follows the trend of the latest guidance, and is a little to the west of the previous one through 36 hours. The official forecast is close to the latest HCCA and TVCA consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 33.7N 43.7W 35 KT 40 MPH 12H 14/0000Z 33.1N 44.1W 35 KT 40 MPH 24H 14/1200Z 32.2N 44.4W 40 KT 45 MPH 36H 15/0000Z 31.9N 44.0W 40 KT 45 MPH 48H 15/1200Z 32.5N 42.6W 40 KT 45 MPH 72H 16/1200Z 35.5N 38.5W 35 KT 40 MPH 96H 17/1200Z...DISSIPATED $$ Forecaster Brennan

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Tropical Storm Isaac Forecast Discussion Number 23

2018-09-13 10:59:17| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 13 2018 142 WTNT44 KNHC 130859 TCDAT4 Tropical Storm Isaac Discussion Number 23 NWS National Hurricane Center Miami FL AL092018 500 AM AST Thu Sep 13 2018 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. 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 12H 13/1800Z 15.5N 61.8W 35 KT 40 MPH 24H 14/0600Z 15.5N 64.8W 35 KT 40 MPH 36H 14/1800Z 15.5N 67.8W 35 KT 40 MPH 48H 15/0600Z 15.4N 70.4W 35 KT 40 MPH 72H 16/0600Z 15.6N 74.8W 35 KT 40 MPH 96H 17/0600Z 16.9N 78.5W 30 KT 35 MPH 120H 18/0600Z...DISSIPATED $$ Forecaster Zelinsky

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Subtropical Storm Joyce Forecast Discussion Number 3

2018-09-13 10:58:45| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 13 2018 024 WTNT45 KNHC 130858 TCDAT5 Subtropical Storm Joyce Discussion Number 3 NWS National Hurricane Center Miami FL AL102018 500 AM AST Thu Sep 13 2018 Joyce is still co-located with an upper-level low, and the most recent TAFB intensity classification was still a subtropical type. Furthermore, there has been no discernible increase of convection near the center of the cyclone, and cloud tops have actually warmed over the past few hours. Therefore, Joyce is still classified as subtropical for this advisory. Since the cyclone hasn't changed much in organization, the initial intensity has been held at 40 kt based on the TAFB subtropical intensity estimate, but this is still possibly generous. Joyce's struggle to maintain deep convection bodes poorly for its intensification prospects, considering that the shear is forecast by the global models to increase considerably over the next few days. SHIPS, LGEM, and the global models continue to show Joyce changing little in strength over the next few days until the system dissipates by early next week. The HWRF and HMON models are still outliers in showing substantial intensification, but even these models aren't quite as aggressive in strengthening Joyce as they were previously. The NHC forecast continues to favor the statistical guidance and is just below the intensity consensus at most forecast hours because it is frankly hard to believe that Joyce will become a hurricane in 48 h based on its current appearance. Joyce is still moving slowly southwestward with an initial motion of 220/5 kt. The global models remain in fairly good agreement that Joyce will move slowly southwestward for the next day or so as the storm remains in light steering flow. A turn toward the south will likely occur as Helene approaches from the southeast, followed by a quick turn northeastward once the larger tropical cyclone passes by. Joyce should then accelerate quickly northeastward ahead of a mid-latitude trough before dissipating southwest of the Azores. Little change was made to the official track forecast which is near the middle of the various consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 33.8N 43.1W 40 KT 45 MPH 12H 13/1800Z 33.3N 43.8W 40 KT 45 MPH 24H 14/0600Z 32.6N 44.2W 45 KT 50 MPH 36H 14/1800Z 32.0N 44.0W 45 KT 50 MPH 48H 15/0600Z 32.1N 43.2W 45 KT 50 MPH 72H 16/0600Z 34.6N 39.4W 40 KT 45 MPH 96H 17/0600Z 37.0N 32.5W 40 KT 45 MPH 120H 18/0600Z...DISSIPATED $$ Forecaster Zelinsky

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