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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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