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Tropical Depression Thirteen Forecast Discussion Number 3
2020-08-20 16:52:11| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Aug 20 2020 000 WTNT43 KNHC 201452 TCDAT3 Tropical Depression Thirteen Discussion Number 3 NWS National Hurricane Center Miami FL AL132020 1100 AM AST Thu Aug 20 2020 The organization of the depression has not changed much overnight or this morning. An area of convection has persisted near the estimated center, with some banding noted over the northwestern portion of the circulation. An earlier SSMIS overpass was very helpful in locating the center of what appears to be a small circulation. A very recent ASCAT overpass has also revealed a small circulation that is weak on the southeastern side, but with winds near tropical storm strength to the north of the center. The ASCAT data along with subjective Dvorak classification from TAFB and SAB support maintaining the 30-kt initial intensity. The depression continues to move briskly west-northwestward or 290/18 kt. The track forecast philosophy has not changed from before. A subtropical ridge over the central Atlantic is forecast to build westward and strengthen over the next several days. This pattern is expected to keep the cyclone on a west-northwestward heading throughout the forecast period. The dynamical models continue to agree on this overall scenario, but there some differences in both forward speed and how close it gets to the Greater Antilles. In general, the models that indicate a stronger cyclone favor a more northern track, while those which depicted a weaker system are along the southern and faster side of the envelope. The latest consensus aids are little north of the previous track, and the new NHC forecast lies between the HFIP corrected consensus and the TVCA multi-model consensus. This is slightly north of the previous advisory, and not far from the GFS ensemble mean. The environment consisting of light to moderate vertical wind shear is expected to allow for gradual strengthening over the next few days, and the NHC forecast calls for the system to become a tropical storm later today or tonight. The upper-level wind pattern is expected to remain favorable in the latter portion of the forecast period, and if there is minimal land interaction, a faster rate of strengthening is possible at that time. The NHC intensity foreast now shows the system becoming a hurricane by 96 hours, but it is a little lower than the consensus aids at days 4 and 5 due to uncertainty in how much the system will interact with the Greater Antilles. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands by Friday night, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday. 2. There is a risk of tropical storm conditions in the Virgin Islands and Puerto Rico Friday night and Saturday and Tropical Storm Watches could be required for these islands later today. Interests there should closely monitor the progress of this system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some storm surge, rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 16.0N 52.0W 30 KT 35 MPH 12H 21/0000Z 17.0N 54.7W 35 KT 40 MPH 24H 21/1200Z 18.0N 58.3W 40 KT 45 MPH 36H 22/0000Z 18.9N 61.6W 45 KT 50 MPH 48H 22/1200Z 19.6N 64.7W 50 KT 60 MPH 60H 23/0000Z 20.4N 68.1W 55 KT 65 MPH 72H 23/1200Z 21.5N 71.8W 60 KT 70 MPH 96H 24/1200Z 24.0N 78.6W 65 KT 75 MPH 120H 25/1200Z 27.0N 83.4W 65 KT 75 MPH $$ Forecaster Brown
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