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Tropical Depression Grace Forecast Discussion Number 15
2021-08-17 04:44:52| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Mon Aug 16 2021 000 WTNT42 KNHC 170244 TCDAT2 Tropical Depression Grace Discussion Number 15 NWS National Hurricane Center Miami FL AL072021 1100 PM EDT Mon Aug 16 2021 IR and microwave imagery during the past few hours indicate that Grace's convective organization has improved a little since this afternoon. However, it is currently unknown if that has translated to a better-defined surface wind field. Dvorak intensity estimates from TAFB and SAB at 00Z supported tropical storm strength, and it is possible Grace has redeveloped sustained winds of that magnitude. A hurricane hunter aircraft is scheduled to investigate Grace in a few hours, so it seems prudent to avoid making a big change before the plane gets there. Grace is therefore conservatively maintained as a 30 kt depression for this advisory. Regardless of Grace's exact intensity right now, the immediate threat is still torrential rainfall across Hispaniola overnight, which will likely cause severe flooding in some locations. The initial motion estimate is 280/12 kt. Confidence in the track forecast has increased considerably during the past 24 h and the track model spread is low. A mid-level ridge is forecast to build over the Gulf of Mexico during the next several days, which should keep Grace on a generally westward or west-northwestward track through day 5. Once Grace moves away from Haiti, it should remain over water until it reaches the Yucatan peninsula late Wednesday or early Thursday. Very little change was made to the official track forecast, which is based primarily on the multi-model consensus TVCN. Now that Grace is forecast to avoid all major land masses for the next couple of days, all of the intensity guidance calls for some strengthening to occur. That said, with the exception of the COAMPS-TC model, the intensity guidance as a whole shows a slower rate of strengthening than it did 6 h ago. The NHC forecast is now very near the IVCN intensity consensus throughout the forecast, including over the Gulf of Mexico where it continues to show Grace reaching hurricane strength. Despite the slightly lower guidance for this forecast, it still can not be ruled out that Grace will reach hurricane strength over the western Caribbean. When the hurricane hunter aircraft reaches Grace in a couple hours and provides more information about the current organization of cyclone, we should have a better feel for how quickly it could intensify prior to reaching the coast of Mexico. Key Messages: 1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, Jamaica, the Cayman Islands, and the northern Yucatan Peninsula may lead to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti and the Dominican Republic. 2. Tropical storm conditions are possible over portions of Hispaniola overnight, and over Jamaica on Tuesday. Tropical storm conditions are expected over portions of the southern coast of Cuba on Tuesday, spreading westward to the Cayman Islands and possibly other portions of the southern coast of Cuba Tuesday evening through Wednesday morning. 3. There is a increasing risk of wind, rainfall, and storm surge impacts on the Yucatan Peninsula of Mexico Wednesday night and Thursday. Interests there areas should monitor the progress of Grace and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 17/0300Z 18.2N 73.8W 30 KT 35 MPH 12H 17/1200Z 18.6N 75.7W 35 KT 40 MPH 24H 18/0000Z 19.1N 78.8W 40 KT 45 MPH 36H 18/1200Z 19.7N 82.0W 45 KT 50 MPH 48H 19/0000Z 20.3N 85.1W 50 KT 60 MPH 60H 19/1200Z 21.0N 88.2W 50 KT 60 MPH...INLAND 72H 20/0000Z 21.6N 91.3W 55 KT 65 MPH...OVER WATER 96H 21/0000Z 22.2N 95.9W 65 KT 75 MPH 120H 22/0000Z 22.5N 100.0W 40 KT 45 MPH...INLAND $$ Forecaster Zelinsky
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