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Tropical Depression Grace Forecast Discussion Number 11
2021-08-16 04:41:37| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Aug 15 2021 000 WTNT42 KNHC 160241 TCDAT2 Tropical Depression Grace Discussion Number 11 NWS National Hurricane Center Miami FL AL072021 1100 PM AST Sun Aug 15 2021 An Air Force Reserve Hurricane Hunter Aircraft investigated Grace earlier this evening and found maximum flight level winds just less than 40 kt at 925 mb, which supports Grace's estimated 30 kt intensity. The plane also found that the depression still has a slightly elongated but closed surface circulation. During the past few hours, satellite imagery has shown a slight increase in convective banding features and associated heavy rain associated with Grace. That activity highlights the primary threat from Grace during the next 24 hours: prolonged heavy rainfall that could lead to flash and urban flooding along with the potential for mudslides over Puerto Rico and Hispaniola. The forecast for Grace is incredibly challenging. Imminent interactions with the high terrain of Hispaniola and Cuba could cause Grace to dissipate as soon as Monday evening. However, a track south of Cuba, as shown by recent runs of the GFS and COAMPS-TC, may allow Grace to maintain its tropical cyclone status and possibly even intensify. The HWRF even shows it becoming a hurricane over the western Caribbean, with the caveat that the model has produced several poor forecasts for Grace thus far. Although it is not explicitly forecast, slight intensification is still possible tonight or tomorrow morning before the center of Grace moves inland. After that time, the NHC forecast assumes Grace will continue as a tropical depression through 72 h. Once/if Grace makes it to the western Caribbean Sea and Gulf of Mexico in 3-4 days, it could have an opportunity to reorganize and intensify, and this is again shown in the official intensity forecast. That said, users are encouraged to not focus on the exact track or intensity forecasts at days 4 and 5. The track guidance has shifted south for this advisory, and generally calls for Grace to move westward to west-northwestward through the forecast period. The official track forecast has been shifted a little south once again, but is north of the most recent multi-model consensus. Key Messages: 1. Heavy rainfall across Puerto Rico, the Dominican Republic, and Haiti may lead to flash, urban, and small stream flooding, along with the potential for mudslides. 2. Tropical storm conditions are possible over portions of Hispaniola Monday and Monday night. 3. There is a risk of some wind and rainfall impacts across Cuba beginning Tuesday morning, but forecast uncertainty is much higher than usual. Interests there areas should monitor the progress of Grace and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 16/0300Z 17.3N 68.6W 30 KT 35 MPH 12H 16/1200Z 17.7N 70.3W 30 KT 35 MPH 24H 17/0000Z 18.7N 72.8W 30 KT 35 MPH 36H 17/1200Z 19.5N 75.3W 30 KT 35 MPH 48H 18/0000Z 20.4N 78.1W 30 KT 35 MPH 60H 18/1200Z 21.2N 81.0W 30 KT 35 MPH 72H 19/0000Z 22.2N 84.0W 30 KT 35 MPH...INLAND 96H 20/0000Z 23.5N 89.0W 35 KT 40 MPH...OVER WATER 120H 21/0000Z 24.5N 93.5W 45 KT 50 MPH $$ Forecaster Zelinsky
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