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Tropical Storm Isaias Forecast Discussion Number 22
2020-08-02 16:59:10| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Sun Aug 02 2020 000 WTNT44 KNHC 021458 TCDAT4 Tropical Storm Isaias Discussion Number 22 NWS National Hurricane Center Miami FL AL092020 1100 AM EDT Sun Aug 02 2020 Another recent burst of deep convection has recently developed northeast of the center, which has resulted in Isaias making a slight northward jog. An impressive mid-level circulation has developed within the strongest thunderstorm cluster, along with rare reflectivity values of 55-60 dBZ for a tropical cyclone. The Melbourne, Florida, Doppler weather radar has been indicating patches of velocity values of 65-66 kt at around 10,000 ft just north of the center, which equates to 58-59 kt surface winds. For now, the intensity will remain at 55 kt since previous convective bursts have not persisted for more than a couple of hours at best. Another Air Force Reserve Hurricane Hunter aircraft is scheduled to investigate Isaias later today. Radar and aircraft fixes indicate that Isaias is now moving toward the north-northwest or 340/07 kt. A slow north-northwestward motion should continue for the next 24 hours or so as Isaias moves into a weakness that has developed in the Bermuda-Azores ridge over north Florida and off the Georgia coast seen in 02/1200Z upper-air data. A turn toward the north is forecast to occur by all of the global models by Monday morning, followed by a faster forward motion toward the northeast by Monday afternoon and evening when the cyclone will be influenced by southwesterly steering flow ahead of a strong eastward-moving mid- to upper-level trough. Continued northeastward acceleration across the mid-Atlantic and New England states is expected on days 3 and 4. The global models continue to show little cross-track difference, but still have significant along-track differences with the GFS being the fastest and the ECMWF being the slowest . Since the preponderance of the model guidance is slower than the GFS solution, the new official forecast track closely follows the consensus models TVCA and HCCA, and lies near the previous advisory track after 12 h. Isaias will remain over warm Gulfstream waters where water temperatures are near 30 deg C. Despite very unfavorable vertical shear conditions of 25-30 kt the past couple of days, the cyclone has managed to hold together, which is an indication that the system has a deep, well-formed vertical circulation. While some slight intensification is possible if the shear decreases, the official forecast calls for the intensity to remain steady until landfall occurs in the Carolinas in 36 hours or so. Some baroclinic interaction with the right-rear quadrant of an anticyclonically curved jet streak is expected to hold the intensity a little above what would normally be expected for a post-landfall tropical cyclone in 48-72 hours. The NHC intensity forecast is similar to the HFIP corrected consensus model and the IVCN intensity consensus model. Key Messages: 1. Tropical storm conditions will spread northward along the east coast of Florida within the warning area through early Monday and will reach the coasts of Georgia, South Carolina, and southern North Carolina within the warning area Monday and early Tuesday. 2. Dangerous storm surge is possible from Edisto Beach South Carolina to Cape Fear North Carolina where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. A Storm Surge Warning may be needed for a portion of this area later today, and residents there should follow advice given by local emergency officials. 3. Heavy rainfall from Isaias will continue to result in potentially life-threatening flash flooding in the Northwest Bahamas through tonight. Flash and urban flooding, some of which may be significant in the coastal Carolinas and Virginia, is expected through midweek along and near the path of Isaias along the U.S. East Coast. Widespread minor to isolated moderate river flooding is possible across portions of the Carolinas and Mid-Atlantic. 4. A Tropical Storm Watch is in effect for portions of the North Carolina coast, where tropical storm conditions are possible on Tuesday. Additional watches and warnings will likely be issued later today as Isaias is expected to move northward near or over the mid-Atlantic and northeast coasts Tuesday and Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 26.9N 79.6W 55 KT 65 MPH 12H 03/0000Z 28.0N 80.1W 55 KT 65 MPH 24H 03/1200Z 29.6N 80.5W 55 KT 65 MPH 36H 04/0000Z 32.2N 79.8W 55 KT 65 MPH 48H 04/1200Z 35.7N 77.7W 50 KT 60 MPH...INLAND 60H 05/0000Z 40.1N 74.3W 45 KT 50 MPH...INLAND 72H 05/1200Z 44.6N 70.4W 40 KT 45 MPH...INLAND 96H 06/1200Z 51.7N 63.2W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 07/1200Z...DISSIPATED $$ Forecaster Stewart
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