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Hurricane Isaias Forecast Discussion Number 14
2020-07-31 17:01:08| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Fri Jul 31 2020 000 WTNT44 KNHC 311501 TCDAT4 Hurricane Isaias Discussion Number 14 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Fri Jul 31 2020 After a brief convective hiatus earlier this morning, a small burst of deep convection with cloud tops as cold as -80C to -85C have developed over the previously exposed low-level center. Dry downslope air coming off of Hispaniola wrapped into the center, eroding the inner-core convection somewhat. However, a surge of moist southwesterly inflow moving through the Windward Passage now appears to be fueling the recent increase in convection. The upper-level outflow has improved somewhat in the western semicircle but still remains restricted due to some modest southwesterly vertical wind shear. The initial intensity has been lowered to 65 kt based on 700-mb flight-level winds of near 70 kt and an increase in the central pressure of 992 mb, which is based on a dropsonde report of 995 mb with 26-kt surface winds. Isaias is continuing on a steady northwestward motion of 305/14 kt. This motion is expected to continue for the next 36 hours or so as the hurricane rounds the southwestern periphery of the Bermuda-Azores High. A gradual turn to the north-northwest and north is expected by 48-60 hours due to a break in the ridge developing in response to a central U.S. shortwave trough digging into the southeastern United States. The timing and strength of this trough will determine how far west Isaias moves before the hurricane turns northward. The 06Z UKMET and and GFS have shifted westward, closer to the Florida coast, which is similar to the westward shift seen in the 00Z ECMWF model fields. By 72 hours, the hurricane is forecast to begin accelerating northeastward, possibly passing over eastern North Carolina by day 4 and across eastern New England on day 5. As a result of the westward shift in the latest model guidance, the new NHC forecast track has also been shifted farther west closer to the southeastern U.S. coast, and lies a little to the west of the consensus models TVCA and NOAA-HCCA. The westward shift in the track forecast has required the issuance of a Hurricane Watch for portions of the Florida east coast. Although some slight weakening has occurred, radar data from the aircraft and the Bahamas radar indicate about a 60-percent eyewall has formed in the northeastern semicircle, which is an indication that the cyclone is trying to reorganize. As a result, strengthening is still expected during the next day or so, especially tonight and Saturday morning during the convective maximum period when the hurricane will be moving over the Gulf Stream where SSTs are 30C or warmer and while the vertical remains reasonably low. Increasing southwesterly shear could cause a gradual decrease in intensity over the weekend. The new official intensity forecast is similar to the previous advisory, and is a little above the available model guidance out of respect for continuity. 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 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. Preparations to protect life and property should be rushed to completion. 3. Tropical storm conditions are expected with hurricane conditions possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Warning and Hurricane Watch are in effect. Storm surge watches or warnings could be needed for part of this area this afternoon. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge beginning late this weekend along the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. 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 31/1500Z 21.7N 74.5W 65 KT 75 MPH 12H 01/0000Z 23.0N 76.1W 70 KT 80 MPH 24H 01/1200Z 24.8N 78.0W 80 KT 90 MPH 36H 02/0000Z 26.3N 79.2W 80 KT 90 MPH 48H 02/1200Z 27.7N 79.8W 75 KT 85 MPH 60H 03/0000Z 29.2N 80.0W 70 KT 80 MPH 72H 03/1200Z 31.6N 79.5W 65 KT 75 MPH 96H 04/1200Z 36.4N 76.2W 55 KT 65 MPH 120H 05/1200Z 43.3N 69.0W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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