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Tropical Storm Isaias Forecast Discussion Number 21
2020-08-02 10:57:01| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Aug 02 2020 000 WTNT44 KNHC 020856 TCDAT4 Tropical Storm Isaias Discussion Number 21 NWS National Hurricane Center Miami FL AL092020 500 AM EDT Sun Aug 02 2020 The burst of deep convection that developed to the northeast of the center last evening has not resulted in any significant improvement in the structure of Isaias according to recent reconnaissance aircraft data and radar imagery. The tropical cyclone is still being adversely affected by about 25 kt of 850-200 mb vertical wind shear. An Air Force Reserve reconnaissance aircraft that has provided several center fixes overnight has found peak 850-mb flight-level winds of 67 kt, and peak believable SFMR winds of 51 kt. These data, along with an earlier ASCAT overpass, indicate that Isaias' intensity is 55 kt. In addition, both the aircraft and ASCAT data suggest that the wind field over the western portion of the circulation is somewhat smaller than previous estimated. Since Isaias has not shown any signs of re-organizing overnight, it appears that the window of opportunity for it to re-strengthen is closing. The SHIPS guidance and global model fields do not show the shear abating during the next 24-36 hours while Isaias moves near the Florida coast. As a result, the new NHC intensity forecast shows little change in strength during this time. Later in the period, gradual weakening should occur as the storm moves northward along the east coast of the United States. The NHC intensity forecast is in good agreement with the HFIP corrected consensus model and the latest multi-model intensity consensus. Recent aircraft fixes and radar data show that Isaias is moving northwestward or 325/8 kt. A slow northwestward to north-northwestward motion around the western periphery of a deep-layer ridge centered near Bermuda should take the center of Isaias very near the east coast of Florida through Monday. After that, the tropical cyclone should begin to accelerate north-northeastward to northeastward as a trough slides eastward into the eastern United States. The dynamical models are tightly clustered, and the NHC track forecast lies between the typically reliable GFS and ECMWF models. The reduction in the initial and predicted intensity of Isaias has necessitated changes in warnings along the east coast of Florida. The Hurricane Warning along the east-central coast of Florida has been replaced with a Tropical Storm Warning. Key Messages: 1. Tropical storm conditions will continue across portions of the northwestern Bahamas through today. 2. Tropical storm conditions are expected to spread northward along the east coast of Florida within the warning area through early Monday. These conditions are expected to spread northward along the coast of Georgia and South Carolina within the warning area on Monday. 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 across portions of the northwestern Bahamas, and flash and urban flooding, especially in low-lying and poorly drained areas, along the East Coast of the United States. Minor to isolated moderate river flooding is possible across portions of the Carolinas and Mid-Atlantic this week. 5. Tropical storm watches are in effect for portions of the coast of South and North Carolina. Additional watches and warnings will likely be issued later today as Isaias is expected to move northward near or over the southeast and mid-Atlantic coasts over the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 26.3N 79.5W 55 KT 65 MPH 12H 02/1800Z 27.4N 80.1W 55 KT 65 MPH 24H 03/0600Z 28.8N 80.6W 55 KT 65 MPH 36H 03/1800Z 30.7N 80.6W 55 KT 65 MPH 48H 04/0600Z 33.7N 79.1W 55 KT 65 MPH...INLAND 60H 04/1800Z 37.8N 76.4W 50 KT 60 MPH...INLAND 72H 05/0600Z 42.9N 72.4W 40 KT 45 MPH...INLAND 96H 06/0600Z 50.0N 65.0W 30 KT 35 MPH...POST-TROP/INLAND 120H 07/0600Z...DISSIPATED $$ Forecaster Brown
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