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| #939902 (Received by flhurricane at: 10:44 AM 13.Sep.2018) 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 |
