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Tropical Storm Isaias Forecast Discussion Number 10
2020-07-30 22:55:23| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Thu Jul 30 2020 000 WTNT44 KNHC 302055 TCDAT4 Tropical Storm Isaias Discussion Number 10 NWS National Hurricane Center Miami FL AL092020 500 PM EDT Thu Jul 30 2020 Surface observations in the Dominican Republic indicate that the poorly defined center of Isaias moved over the southeastern portion of Hispaniola around 1600 UTC. There is currently a significant concentration of convection near the mid-level center that is located along the northern coast of the island, and recent surface observations suggest that a new center may be forming near the area of mid-level rotation seen in satellite imagery. The advisory position has been placed between the previous estimated center location and the mid-level center until it becomes more clear that reformation has occurred. Earlier ASCAT data that arrived shortly after the previous advisory and reconnaissance aircraft data from this morning supported an intensity of 45-50 kt, and since there has no significant degradation the overall organization since that time, the initial intensity remains 50 kt. The central pressure of 999 mb is based on a surface observation of 1001 mb and 25 kt of wind at Puerto Plata in the Dominican Republic. The initial motion estimate is a somewhat uncertain 310/17 kt. The track forecast reasoning has not changed since the previous advisory. The tropical storm should continue on a general northwest heading during the next 24 to 36 hours along the southern side of a subtropical ridge. After that time, a trough moving into the east-central United States should cause Isaias to turn north-northwestward and northward as the western portion of the ridge erodes. By early next week, Isaias is expected to turn northeastward and accelerate ahead of the aforementioned trough. The 1200 UTC dynamical model guidance has shifted eastward and now that Isaias is expected to become a stronger and deeper cyclone, that also favors a more eastward track. The updated official forecast is a blend of the various consensus aids. Since the new center is still in its formative stage some additional shifts in the track may occur. In addition, strong winds and heavy rainfall will extend far from the center. Visible imagery has shown significant banding near the mid-level circulation. Assuming that the center reforms near that feature and moves away from Hispaniola tonight, strengthening is expected during the next 24-36 hours and Isaias is now forecast to become a hurricane. After that time, there are mixed signals regarding the amount of southwesterly vertical wind shear and the models generally do not show much additional strengthening. Therefore, the NHC intensity forecast shows a leveling off in intensity after 48 h. It still needs to be stressed that that there is a higher than usual amount of uncertainty in the intensity forecast. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across Puerto Rico, the Dominican Republic, northern Haiti, Turks and Caicos and the Bahamas. 2. Hurricane conditions and dangerous storm surge are expected in portions of the central and northwest Bahamas late Friday and Saturday. Preparations to protect life and property should be rushed to completion. Tropical storm conditions will continue to spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the southeast Bahamas tonight through Friday, and Tropical Storm Warnings are in effect for these areas. 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch has been issued. While storm surge watches are not currently needed for this area, they may be required tonight or early tomorrow if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect eastern Florida this weekend, potentially resulting in isolated flash and urban flooding. 4. There is a risk of impacts from winds, heavy rainfall, and storm surge this weekend along the Florida east coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain and it is too soon to determine the magnitude and location of these potential impacts, but 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 30/2100Z 19.5N 70.6W 50 KT 60 MPH...INLAND 12H 31/0600Z 20.8N 72.7W 55 KT 65 MPH...OVER WATER 24H 31/1800Z 22.7N 75.1W 60 KT 70 MPH 36H 01/0600Z 24.5N 77.1W 65 KT 75 MPH 48H 01/1800Z 26.1N 78.5W 65 KT 75 MPH 60H 02/0600Z 27.7N 79.5W 65 KT 75 MPH 72H 02/1800Z 29.2N 79.7W 65 KT 75 MPH 96H 03/1800Z 33.3N 78.0W 65 KT 75 MPH 120H 04/1800Z 40.0N 71.0W 60 KT 70 MPH $$ Forecaster Brown
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