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Tropical Storm Fred Forecast Discussion Number 6
2021-08-11 04:55:50| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Tue Aug 10 2021 000 WTNT41 KNHC 110255 TCDAT1 Tropical Storm Fred Discussion Number 6 NWS National Hurricane Center Miami FL AL062021 1100 PM AST Tue Aug 10 2021 Data from satellites, the San Juan NOAA Doppler weather radar, and an Air Force Reserve Unit reconnaissance aircraft indicate that the low-level circulation has become better defined since the previous advisory and,thus, the disturbance has been upgraded to Tropical Storm Fred. Although the surface and low-level circulations are vertically aligned, radar data indicate that the mid-level circulation is still tilted about 40 n mi to the south. However, that mid-level feature has been steadily gaining ground from its earlier 100-nmi southeastward displacement. Radar Doppler velocity data, aircraft 850-mb flight-level wind data of 43 kt and SFMR surface winds of 33 kt, plus earlier surface observations of sustained 33-kt winds, support an initial intensity of 35 kt. The initial motion estimate is westward or 270/15 kt. However, this should be a short-term motion overnight due to interaction with the mountainous terrain of Puerto Rico to the north. By early Wednesday morning, Fred is expected to resume a west-northwestward motion and pass through the Mona Passage, reaching the eastern Dominican Republic shortly after sunrise. A strong subtropical ridge to the north of the cyclone is forecast by the global models to basically remain static across the central and western Atlantic Ocean for the next five days. Given this pattern, Fred is expected to maintain a west-northwestward motion through 96 hours, remaining just offshore the north coast of Haiti and Cuba during that time. After emerging over the southeastern Gulf of Mexico on day 4, Fred is forecast to gradually turn more north-northwestward as the cyclone rounds the southwestern periphery of the aforementioned ridge. The latest NHC track forecast models strongly support this track scenario, although there was a slight northward shift in the guidance envelope on this cycle. The new official track forecast was shifted a little north of the previous one through 72 hours, and then lies down the middle of the tightly packed track consensus models. Conditions remain favorable for some strengthening before Fred reaches Hispaniola in about 12 h, but the intensity likely won't be much stronger than 40-45 kt at landfall. Interaction with the mountainous terrain of Hispaniola on Wednesday should result in some weakening, but tropical-storm-force winds could occur along the the north coast of the island if the center shifts a little more to right of the forecast track. After emerging back over water by late Wednesday, only slow strengthening is forecast in the 36-60-h time period due to westerly vertical wind shear of about 20 kt expected to affect the cyclone. On days 4 and 5, however, Fred is expected to be moving over warmer sea-surface temperatures and into a much weaker wind shear regime, which would favor more robust strengthening. As a result, some of the regional and statistical- dynamical intensity models bring Fred near or to hurricane strength during that time. However, there is uncertainty Fred's structure after interaction with Hispaniola and possibly Cuba, and continued disagreement between the global models on how the upper-level wind flow pattern near the cyclone will evolve, which ranges from favorable anticyclonic outflow to strong southwesterly shear. Given these factors, the NHC intensity forecast was only nudged slightly higher than the previous advisory, and lies between the consensus models HCCA and IVCN, and the SHIPS and LGEM models. KEY MESSAGES: 1. Tropical storm conditions are expected in portions of the U.S. Virgin Islands and Puerto Rico tonight, and are expected to begin in the Dominican Republic Wednesday morning. Tropical storm conditions are possible in northern Haiti and the southeastern Bahamas by late Wednesday. 2. Heavy rainfall could lead to flash, urban, and small stream flooding, along with possible rapid river rises and the potential for mudslides across the U.S. Virgin Islands, Puerto Rico, and the Dominican Republic. The greatest threat for flooding impacts will be across eastern and southern Puerto Rico. 3. There is a risk of wind and rainfall impacts elsewhere in portions of Hispaniola, the Bahamas, and Cuba during the next several days, although the forecast is more uncertain than usual since Fred is forecast to interact with Hispaniola on Wednesday. Interests in these areas should monitor Fred's progress and updates to the forecast. 4. There is a risk of wind and rainfall impacts in Florida beginning Friday in the Keys and spreading northward through portions of the Peninsula and the Panhandle this weekend. However, it is too soon to determine the exact timing, location, and magnitude of any potential impacts. Interests throughout Florida should monitor Fred's progress and check updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 17.4N 66.8W 35 KT 40 MPH 12H 11/1200Z 18.6N 68.9W 40 KT 45 MPH...INLAND 24H 12/0000Z 19.9N 71.4W 35 KT 40 MPH...INLAND 36H 12/1200Z 20.9N 73.7W 35 KT 40 MPH...OVER WATER 48H 13/0000Z 21.8N 76.0W 35 KT 40 MPH 60H 13/1200Z 22.8N 78.0W 35 KT 40 MPH 72H 14/0000Z 23.8N 79.9W 40 KT 45 MPH 96H 15/0000Z 26.5N 82.8W 45 KT 50 MPH 120H 16/0000Z 29.3N 84.6W 55 KT 65 MPH $$ Forecaster Stewart
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