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Hurricane Isaias Forecast Discussion Number 18
2020-08-01 17:00:22| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Sat Aug 01 2020 000 WTNT44 KNHC 011500 TCDAT4 Hurricane Isaias Discussion Number 18 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Sat Aug 01 2020 Data from an Air Force Reserve reconnaissance aircraft around 1410 UTC indicated that the center of the eye of Isaias was located along the southern coast of northern Andros Island about 15 nmi south-southwest of Andros Town. The eye appearance in Bahamas and aircraft radar data, along with the visual reports from the flight crew, has gone from nearly closed a few hours ago to open in the southwestern quadrant more recently. However, the diameter of the eye has been has been holding steady between 20-22 nmi, an indication that Isaias has been able to fight off some modest southwesterly wind shear. The initial intensity has been lowered to 70 kt based on maximum 700-mb flight-level winds of 77 kt, which equals about 70-kt surface wind speed, and the central pressure fluctuating between 987-990 mb. The initial motion estimate remains 315/10 kt. The latest NHC model guidance is now tightly packed about the previous 18-h worth of official track forecasts, and as a result, no significant changes were made to the previous advisory track. The global and regional models have come into much better agreement compared to 24 hours ago in taking Isaias northwestward slowly for the next 36 h or so, and moving the center near or keeping it just offshore the east-central Florida coast. By 48 hours, the hurricane is expected to turn northward around the western portion of the Bermuda-Azores ridge that will slowly be eroded by an approaching mid-level shortwave trough currently situated over the central United States. By 60 h and beyond, the aforementioned trough is expected to gradually accelerate Isaias northeastward near or along the coast from South Carolina to New England. The new NHC track forecast is basically an extension of the previous one, and lies very close to an average of the simple consensus model TVCA, and the corrected- consensus models NOAA-HCCA and FSSE. In the near term, Isaias could weaken a little bit this afternoon while passing over northern Andros Island. However, the still impressive vertical structure of the cyclone should allow for some re-strengthening after the center moves back over the warm Gulf Stream by this evening. The current SHIPS analyzed westerly vertical wind shear of 25 kt could be too high due to the model incorporating some of the storm's outflow. The models forecast the shear to weaken somewhat over the next 36 h while Isaias is moving over the Gulf Stream, and the 06Z UKMET shows Isaias moving underneath a 200-mb anticyclone, which would normally favor some strengthening. Given all of these factors, the official forecast maintains a steady intensity through Monday night. Slow weakening is forecast when Isais encounters more significant southwesterly vertical wind shear ahead of a strong upper-level trough that will be approaching the U.S. east coast on days 3-5. Key Messages: 1. Hurricane conditions and dangerous storm surge will continue in portions of the northwest Bahamas today and tonight. 2. Hurricane conditions are expected along portions of the Florida east coast by late tonight and Sunday. Preparations to protect life and property should be rushed to completion. 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents there should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash flooding in the Bahamas, and flash urban flooding, especially in low-lying and poorly drained areas, in eastern Florida, and from the Carolinas to the mid-Atlantic and Northeast U.S. Minor river flooding is possible across portions of the Carolinas and Virginia early next week. 5. Tropical storm watches and warnings are in effect for portions of the northeast Florida and southern Georgia coasts. Additional watches and warnings will likely be issued later today and Sunday along the U.S. east coast as the risk of wind, heavy rainfall, and storm surge impacts continues to increase. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 24.7N 77.9W 70 KT 80 MPH 12H 02/0000Z 25.7N 78.8W 70 KT 80 MPH 24H 02/1200Z 27.1N 79.7W 70 KT 80 MPH 36H 03/0000Z 28.5N 80.3W 70 KT 80 MPH 48H 03/1200Z 30.1N 80.4W 65 KT 75 MPH 60H 04/0000Z 32.6N 79.4W 60 KT 70 MPH 72H 04/1200Z 36.0N 76.9W 50 KT 60 MPH...INLAND 96H 05/1200Z 43.3N 69.7W 45 KT 50 MPH...OVER WATER 120H 06/1200Z 49.0N 57.9W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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