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#1010410 (Received by flhurricane at: 4:59 AM 31.Jul.2020) TCDAT4 Hurricane Isaias Discussion Number 13 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Fri Jul 31 2020 Satellite images indicate that Isaias is maintaining a small central dense overcast, with a new burst of convection recently forming near the center. Earlier aircraft data indicated that the pressure had fallen to 990 mb, although there was little from the aircraft that supported more than the previous advisory's intensity of 70 kt. The hurricane has been moving between west-northwest and northwest during the last several hours, estimated at 305/15 now. Isaias should gradually turn to the north-northwest and north over the weekend as it moves around the western periphery of a weakening western Atlantic subtropical ridge. Guidance is in fairly good agreement over the first couple of days of the forecast, so little change is made at that time, other than a small westward adjustment near the Bahamas. After that time, there is a considerable spread, growing to over 700 miles, in the reliable models on how quickly Isaias moves northeastward near the east coast of the United States. The large spread appears to be related to both how strong the cyclone is by early next week and how much mid-level ridging remains near the Carolinas. There are no obvious reasons to choose either the fast GFS or slow ECMWF right now, so the official forecast will remain close to the previous one and the model consensus. It should be noted that given this large spread, the extended forecast could be subject to large speed/timing changes if either of those solutions become more likely. Strengthening is expected during the next day or so while the hurricane remains over the very warm waters near the Bahamas with reasonably low vertical shear. The forecast for the first 24 hours has been raised from earlier, at the top of the guidance spread. Increasingly southwesterly flow aloft should cause an increase in shear over the weekend, which is forecast by most of the guidance to lead to a gradual decrease in intensity. The details are not very clear at all, especially considering the track uncertainty, but there is a notable chance of a hurricane moving close to the U.S. East coast, so the forecast continues to show that scenario. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect South and east-Central Florida beginning late Friday night, and the eastern Carolinas by early next week, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. Isolated minor river flooding is possible in the Carolinas early next week. 2. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas today and Saturday, and Hurricane Warnings are in effect for these areas. Preparations to protect life and property should be rushed to completion. 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch remains in effect. While storm surge watches are not currently needed for this area, they may be required later today if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect South Florida and east-Central Florida beginning late tonight, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge late this weekend from the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain, and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 20.9N 73.3W 70 KT 80 MPH 12H 31/1800Z 22.3N 75.0W 80 KT 90 MPH 24H 01/0600Z 24.0N 77.0W 85 KT 100 MPH 36H 01/1800Z 25.7N 78.5W 80 KT 90 MPH 48H 02/0600Z 27.3N 79.4W 75 KT 85 MPH 60H 02/1800Z 28.8N 79.6W 75 KT 85 MPH 72H 03/0600Z 30.6N 79.3W 70 KT 80 MPH 96H 04/0600Z 36.5N 75.5W 65 KT 75 MPH 120H 05/0600Z 42.5N 69.0W 55 KT 65 MPH $$ Forecaster Blake