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Tropical Storm Grace Forecast Discussion Number 16
2021-08-17 10:59:57| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Tue Aug 17 2021 000 WTNT42 KNHC 170859 TCDAT2 Tropical Storm Grace Discussion Number 16 NWS National Hurricane Center Miami FL AL072021 500 AM EDT Tue Aug 17 2021 An Air Force Reserve reconnaissance aircraft had difficulty locating the center of Grace early this morning. The last few visible satellite images along with 0200Z ASCAT wind data indicated that a lee-side surface low had formed south of the western tip of Haiti. That feature has gradually been pulling northward closer to the deep convection that has developed near the well-defined mid-level circulation center. The initial position is the midpoint between the recon position in the latest vortex message and the mid-level circulation noted in passive microwave imagery. The aircraft also found some patches of 925-mb flight-level winds of 48-51 kt, which equates to surface winds of 36-38 kt. Reliable SFMR surface winds obtained by the aircraft were also in the 36-38-kt range. Based on these data, Grace was upgraded back to a 35-kt tropical storm at 0600 UTC. Regardless of Grace's exact intensity right now, the immediate threat is still torrential rainfall across western Hispaniola today and over Jamaica this afternoon and tonight, which will likely cause severe flooding in some locations. The initial motion estimate is an uncertain 280/14 kt. A strong mid-level ridge is forecast to build westward across Florida and the Gulf of Mexico during the next several days, which should keep Grace on a westward or west-northwestward track through the 120-h forecast period. Only a very sight southward shift in the previous forecast track was made, which was due primarily to the new estimated southerly initial position, and the new NHC forecast track lies along the northern edge of model guidance envelope. Once Grace passes Jamaica in 12-18 hours, the cyclone will be passing over water temperatures near 30 deg C and into a more divergent upper-level wind flow pattern. These two favorable conditions should allow for Grace to slowly intensify and possibly be near hurricane strength by the time the cyclone reaches the Yucatan peninsula in about 48 hours. Thereafter, Grace will weaken as it pass over Yucatan, and then re-strengthen some over the Bay of Campeche before reaching mainland Mexico in about 96 hours. The NHC intensity forecast is close to a blend of the HCCA and IVCN consensus models. 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 and over Jamaica on today. Tropical storm conditions are expected over portions of the southern coast of Cuba today, spreading westward to the Cayman Islands and possibly other portions of the southern coast of Cuba this 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 17/0900Z 18.2N 75.4W 35 KT 40 MPH 12H 17/1800Z 18.6N 77.6W 40 KT 45 MPH 24H 18/0600Z 19.1N 80.7W 45 KT 50 MPH 36H 18/1800Z 19.7N 83.9W 50 KT 60 MPH 48H 19/0600Z 20.4N 87.0W 60 KT 70 MPH 60H 19/1800Z 21.0N 90.0W 50 KT 60 MPH...INLAND 72H 20/0600Z 21.4N 92.7W 55 KT 65 MPH...OVER WATER 96H 21/0600Z 22.0N 97.2W 60 KT 70 MPH 120H 22/0600Z 22.3N 101.4W 30 KT 35 MPH...INLAND $$ Forecaster Stewart
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