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Hurricane Isaias Forecast Discussion Number 15
2020-07-31 22:50:07| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Fri Jul 31 2020 000 WTNT44 KNHC 312050 TCDAT4 Hurricane Isaias Discussion Number 15 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Fri Jul 31 2020 Deep convection, with occasional overshooting cloud tops of -85C to -90C just north of the center, has continued to develop during the normal diurnal convective minimum period, which is quite impressive. The most recent Air Force Reserve recon flight-level wind data, along with ASCAT surface wind data, indicate that the inner-core and outer wind field have both contracted in size. Furthermore, radar data from the Bahamas and an 1810Z AMSR-2 microwave pass also indicate that a small 10-nmi-wide mid-level eye is forming. The last recon central pressure was 991 mb and the 700 mb height had decreased by 30 meters since the earlier maximum height around 1230Z. These data indicate that Isaias is getting better organized. The initial intensity remains 65 kt based on an earlier 700-mb flight-level wind speed of 72 kt, which reduces to a 65-kt surface wind speed using a 90-percent adjustment factor. The initial motion remains northwestward or 305/12 kt. The 12Z global models have once again made a westward shift due to the ridge to the north of Isaias not weakening as quickly as expected. This is partly due to the ridge being stronger than expected and a shortwave trough over the central United States moving a little slower into the southeastern U.S. than previously indicated. The UKMET and ECMWF explicitly show Isaias making landfall in 36-48 hours along the southeast Florida coast, but appear to weaken the system below hurricane strength. The GFS similarly brings the cyclone close to the southeast and east-central Florida coasts, but also as a somewhat weaker system. In the 48 to 60-hour period, the cyclone is forecast to move slowly north-northwestward and northward through a break in the subtropical ridge extending westward from the Atlantic across Florida and into the northern Gulf of Mexico. By that time, however, Isaias is expected to weaken below hurricane strength due to the combination of strong southwesterly vertical wind shear and interaction the Florida peninsula. Around 72 hours, the cyclone should accelerate northeastward and possibly strengthen some before passing over eastern North Carolina on day 4, and across eastern New England on day 5. The NHC track forecast lies close to a blend of the consensus models TVCA and NOAA-HCCA and is east of the UKMET and ECMWF with the system forecast to be stronger than those models indicate. Due to the westward shift in the NHC forecast track, a Hurricane Warning and Storm Surge Watch have been issued for portions of the Florida east coast. The center of Isaias is now located in the center of an expanding CDO feature. The improved inner-core wind field and aforementioned convective structure, along with very warm SSTs near 30C, should support some strengthening overnight and early Saturday morning. However, increasing southwesterly vertical wind shear is expected to cause a gradual decrease in intensity by Sunday and continue into early next week. The new official intensity forecast is a little lower than the previous advisory and is near the higher end of the intensity guidance. Key Messages: 1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect. 2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. 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 in these areas should follow advice given by local emergency officials. 4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas across south to east-central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week. 5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 22.6N 75.7W 65 KT 75 MPH 12H 01/0600Z 23.9N 77.2W 70 KT 80 MPH 24H 01/1800Z 25.4N 78.8W 75 KT 85 MPH 36H 02/0600Z 26.8N 79.9W 70 KT 80 MPH 48H 02/1800Z 28.3N 80.4W 65 KT 75 MPH 60H 03/0600Z 30.0N 80.4W 60 KT 70 MPH 72H 03/1800Z 32.5N 79.4W 60 KT 70 MPH 96H 04/1800Z 39.0N 74.5W 55 KT 65 MPH 120H 05/1800Z 45.4N 65.9W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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