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Potential Tropical Cyclone Nine Forecast Discussion Number 4
2020-07-29 10:46:56| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Jul 29 2020 000 WTNT44 KNHC 290846 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 4 NWS National Hurricane Center Miami FL AL092020 500 AM AST Wed Jul 29 2020 Satellite and surface data indicate that the system remains a trough of low pressure, elongated from SSW to NNE, with almost all of the strong winds far north of the center position. The most significant curvature in the low-level wind field and on radar are near Dominica now, which has good continuity from the previous advisory, so this feature will continue to be used as the center. A combination of unflagged SFMR winds from an Air Force plane and earlier scatterometer winds support 40 kt as the initial wind speed. The initial motion estimate is about the same as before or 295/20 kt. The ridge to the north of the disturbance is forecast to remain strong for the next 36 h, which keeps the system moving speedily in a general west-northwestward direction just south of the Leeward Islands today, and near or over the Greater Antilles on Thursday. The ridge is forecast to weaken after that time, which should cause the cyclone to slow down, and potentially gain more latitude over the southwestern Atlantic. The model guidance is generally showing a narrow ridge persisting for a bit longer, however, causing a small south and west shift in the new NHC forecast at long range. However, it should be emphasized that this forecast track is highly uncertain until a true center forms. Satellite images indicate that a large burst of convection is occuring near the poorly defined center, which will likely lead to the system becoming a tropical storm later today. Further intensification is possible before landfall in the Dominican Republic on Thursday, assuming the structure continues to improve, and the wind speed forecast is adjusted slightly higher in the near term. The cyclone will probably take some time to recover after moving over the very high mountains of Hispaniola, and some of the guidance after that time shows an increase in southwesterly shear over the Straits of Florida that could limit the potential of the cyclone. Simply put, there are a lot of hurdles in the system's way, so it is best to stay on the conservative side at the moment and continue to stress the large uncertainty after it leaves the Caribbean. Little change was made to the forecast intensity, although the guidance has come down at longer range for many of the models. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands and Puerto Rico today and spreading westward to portions of the Dominican Republic and Haiti on Thursday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and it could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 15.3N 61.3W 40 KT 45 MPH...POTENTIAL TROP CYCLONE 12H 29/1800Z 16.3N 64.1W 45 KT 50 MPH...TROPICAL STORM 24H 30/0600Z 17.8N 67.6W 50 KT 60 MPH 36H 30/1800Z 19.3N 71.0W 40 KT 45 MPH...INLAND 48H 31/0600Z 20.6N 74.0W 40 KT 45 MPH...OVER WATER 60H 31/1800Z 22.1N 76.7W 45 KT 50 MPH 72H 01/0600Z 23.3N 78.7W 50 KT 60 MPH 96H 02/0600Z 25.5N 81.5W 55 KT 65 MPH 120H 03/0600Z 28.5N 83.0W 45 KT 50 MPH $$ Forecaster Blake
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