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Tropical Storm Grace Forecast Discussion Number 6
2021-08-14 22:43:09| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Aug 14 2021 000 WTNT42 KNHC 142043 TCDAT2 Tropical Storm Grace Discussion Number 6 NWS National Hurricane Center Miami FL AL072021 500 PM AST Sat Aug 14 2021 Grace is a poorly organized tropical storm this afternoon. The NOAA Hurricane Hunters were unable to close off a circulation, at least at their flight level of 5000 feet, but dropsonde observations and reports from some of the islands in the Lesser Antilles suggest that there's at least a broad cyclonic circulation at the surface. The various data also indicate that the center has sped up, or re-formed, and is located farther southwest than previously estimated. Based on the aircraft data and earlier ASCAT data, the initial intensity is set at 35 kt. Both the future track and intensity of Grace have a high level of uncertainty. For the track, the forecast is likely to be complicated by the fact that the system doesn't currently have a tight low-level circulation, and the center could always re-form at any time, especially with convection ongoing farther to the north. In addition, the storm has not yet slowed down, and in fact, the initial motion is estimated to be toward the west (275 degrees) at 23 kt. The guidance envelope has made a notable southward shift due to the adjustment of the initial position, and the models insist that Grace will primarily have a west-northwestward heading for much of the forecast period with the speed gradually decreasing during the next 48 hours or so. The NHC track forecast has been shifted southward accordingly, although any re-formations of the center could cause this track to shift again in future advisory cycles. If Grace slows down as forecast--which is obviously not a sure thing--environmental conditions should be conducive to allow for some strengthening before the system reaches the Greater Antilles. The southward adjustment in the official forecast now takes Grace over the Greater Antilles for a longer period of time, and the official intensity forecast is therefore lowered beyond 48 hours. This is a middle-of-the-road solution, and actually lower than most of the intensity guidance. If the forecast track shifts north or south, the system could strengthen further over water. Alternatively, Grace could go the way of Fred and dissipate before the end of the 5-day period. In the end, the exact track of the center and the intensity of the system will likely not be as important as the heavy rainfall that is forecast to fall across the Leeward Islands and the Greater Antilles during the next few days. Key Messages: 1. Tropical storm conditions are expected in portions of the Leeward Islands tonight, and the Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are possible over eastern parts of the Dominican Republic Sunday night and Monday. 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, the Dominican Republic, and Haiti, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides. 3. There is a risk of wind and rainfall impacts across the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, Cuba, and Florida next week, but forecast uncertainty remains higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 15.9N 60.7W 35 KT 40 MPH 12H 15/0600Z 16.5N 63.3W 40 KT 45 MPH 24H 15/1800Z 17.1N 66.1W 45 KT 50 MPH 36H 16/0600Z 17.7N 68.5W 50 KT 60 MPH 48H 16/1800Z 18.3N 70.6W 50 KT 60 MPH...NEAR DOMINICAN REP. 60H 17/0600Z 19.2N 72.8W 40 KT 45 MPH...NEAR HAITI 72H 17/1800Z 20.2N 75.3W 40 KT 45 MPH...OVER CUBA 96H 18/1800Z 22.4N 80.1W 35 KT 40 MPH...OVER CUBA 120H 19/1800Z 24.8N 84.3W 45 KT 50 MPH...OVER WATER $$ Forecaster Berg
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