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Tropical Storm Isaias Forecast Discussion Number 8
2020-07-30 10:52:45| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Jul 30 2020 000 WTNT44 KNHC 300852 TCDAT4 Tropical Storm Isaias Discussion Number 8 NWS National Hurricane Center Miami FL AL092020 500 AM AST Thu Jul 30 2020 Isaias is sending some mixed signals tonight. The 1-min rapid scan data from GOES-16 indicates that the low-level center is likely displaced west of a very intense burst of deep convection on the northeastern side of the circulation. However, the satellite data also shows increased banding features overnight and a more organized central cloud pattern, with recent hints that perhaps a low-level center is trying to re-form closer to the convection. Radar observations from San Juan show 60-65 kt Doppler wind velocities during the past few hours near 5000 ft, so the initial wind speed is raised to 50 kt. Model forecasts are showing a complex evolution of the tropical cyclone during the next day or two. There is good agreement that Isaias will move across Hispaniola later today, and its low-level center will likely become disorganized over the high terrain. However, the strong burst of convection currently near Puerto Rico is associated with a mid-level circulation, which should pass along the north coast of Hispaniola later today. Most of the model guidance suggest that this feature will cause the re-development of a surface center over the northern part of the broader system while the mid-level circulation moves close to the southeastern Bahamas. Afterward, the cyclone would then move northwestward until the weekend, and gradually turn northward and northeastward close to the U.S. East Coast into early next week ahead of a mid-latitude trough. The official track forecast is a little to the east of the previous one and close to the NOAA corrected dynamical model consensus. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola. The intensity forecast is quite tricky. In the short term, Isaias is expected to move across Hispaniola, as the storm's interaction with the mountainous island should cause some weakening and disruption to the circulation. However, as mentioned before, the models suggest that a new center could form, and the environmental conditions would support gradual intensification. The intensity models have been trending higher, and the official forecast is nudged upward accordingly, now showing a peak intensity of 60 kt when the storm is near the coast of Florida and the Southeast U.S. Coast. It should be noted that there are models that show hurricane strength near the U.S. but, given the large amount of uncertainty, it is preferred to stay on the conservative side for now. We should have a better idea of how strong Isaias will become near the U.S. after reconnaissance aircraft sample the storm and after it passes Hispaniola later today. 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 Bahamas. 2. Tropical storm conditions are likely across portions of the the Virgin Islands, and Puerto Rico through this morning and will spread westward to portions of the Dominican Republic, Haiti, and the Turks and Caicos and the Bahamas later today 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 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 couple of days. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 17.2N 67.9W 50 KT 60 MPH 12H 30/1800Z 18.8N 70.2W 45 KT 50 MPH...INLAND 24H 31/0600Z 20.6N 72.9W 45 KT 50 MPH...OVER WATER 36H 31/1800Z 22.3N 75.5W 50 KT 60 MPH 48H 01/0600Z 24.0N 77.9W 55 KT 65 MPH 60H 01/1800Z 25.5N 79.2W 60 KT 70 MPH 72H 02/0600Z 27.3N 80.0W 60 KT 70 MPH 96H 03/0600Z 31.0N 80.0W 60 KT 70 MPH 120H 04/0600Z 37.0N 74.5W 60 KT 70 MPH $$ Forecaster Blake/Cangialosi
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