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Tropical Storm Isaias Forecast Discussion Number 7
2020-07-30 05:01:12| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Jul 29 2020 991 WTNT44 KNHC 300301 TCDAT4 Tropical Storm Isaias Discussion Number 7 NWS National Hurricane Center Miami FL AL092020 1100 PM AST Wed Jul 29 2020 Observations from recent scatterometer passes over the system show that it now has a sufficiently well-defined center to be designated as a tropical cyclone. The current intensity is estimated to be 45 kt, but these winds are currently occurring over the Atlantic waters well to the north and northeast of the center. Since the cyclone is expected to move over Hispaniola on Thursday some weakening is likely within the next 24 hours. However since Isaias has such a broad wind field, the weakening will probably not be as significant as in a typical tropical cyclone with a small radius of maximum winds. Also, a re-formation of the center to the north of Hispaniola may occur. Later in the forecast period some strengthening is likely, although this may be offset by southwesterly wind shear on the order of 20 kt in 2-3 days. The official forecast is close to the intensity model consensus but well below the latest LGEM guidance. The scatterometer data show that the center of the system is south of the previously estimated track, so there is a lot of uncertainty in the initial motion estimate of 285/17 kt. Isaias should move on a west-northwestward to northwestward track on the southern and southwestern side of a mid-tropospheric ridge. In 2-3 days, the system is expected to turn north-northwestward due to a weakness in the ridge and an approaching trough. Later in the forecast period, the trough should cause Isaias to turn toward the northeast. The official track forecast is a little to the east of the previous one and a little west of the simple and corrected dynamical model consensus tracks. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the southeastern Bahamas. 2. Tropical storm conditions are likely across portions of the the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos and the Central Bahamas on Thursday and Friday. 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. While this system could bring some rainfall and wind impacts to portions of Cuba, the northwestern Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 15.8N 67.0W 45 KT 50 MPH 12H 30/1200Z 17.6N 69.3W 45 KT 50 MPH 24H 31/0000Z 19.4N 72.2W 40 KT 45 MPH...INLAND 36H 31/1200Z 21.4N 74.8W 45 KT 50 MPH...OVER WATER 48H 01/0000Z 22.9N 77.3W 50 KT 60 MPH 60H 01/1200Z 24.8N 79.2W 55 KT 65 MPH 72H 02/0000Z 26.6N 80.2W 55 KT 65 MPH...INLAND 96H 03/0000Z 30.0N 80.5W 55 KT 65 MPH...INLAND 120H 04/0000Z 35.0N 77.0W 50 KT 60 MPH...INLAND $$ Forecaster Pasch
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