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Tropical Depression Grace Forecast Discussion Number 12
2021-08-16 10:48:10| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Mon Aug 16 2021 930 WTNT42 KNHC 160848 TCDAT2 Tropical Depression Grace Discussion Number 12 NWS National Hurricane Center Miami FL AL072021 500 AM EDT Mon Aug 16 2021 The structure and organization of Tropical Depression Grace has changed little over the past several hours. An earlier ASCAT-B overpass showed that the circulation is elongated from the NE-SW, with peak winds of 30 kt. An Air Force Reserve Hurricane Hunter Aircraft investigated Grace a few hours ago, and indicated that the cyclone refuses to make the decrease in forward speed and turn to the west-northwest that most of the guidance has been calling for. The aircraft also measured peak flight level winds of 37 kt at 925 mb, which equates to about 28 kt at the surface. Although there were some SFMR vectors of tropical storm force, those winds were sampled in heavy rainfall and were likely not representative of the actual surface winds. Based on this data, the initial intensity is kept at 30 kt for this advisory. Grace is moving toward the west, or 275/13 kt. The forecast models continue to make a shift to the south, as they are coming into very good agreement on maintaining a strong mid-level ridge to the north of the cyclone throughout the forecast period. The latest track forecast was adjusted a little south of the previous one through the first couple of days of the forecast, with a larger shift to the south farther out in time. This forecast track remains to the north of the track consensus, so if the track models remain consistent, additional adjustments to the south will likely be made in the next forecast cycle. The evolution of the forecast track of Grace is having major implications on the future intensity of the cyclone. It is becoming more likely that the system will track over only the southern portion of Hispaniola, which would result in a lesser disruption of Grace's circulation. Furthermore, the more southern track would bring the cyclone over the warm waters of the northwestern Caribbean beginning on Tuesday. The environment over that portion of the Caribbean is not all that hostile, with about 10-15 kt of northerly shear being the main inhibiting factor for intensification. By midweek, Grace may interact with the Yucatan Peninsula, which should limit strengthening or perhaps weaken the system depending on the track. Once over the western Gulf of Mexico, late in the forecast period, some additional strengthening is expected, although the wind shear is forecast to increase slightly during that time frame. The latest NHC intensity forecast was raised slightly to reflect the additional time expected over water. However, this forecast is a little lower than most of the guidance. Key Messages: 1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, Jamaica, and the Cayman Islands 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 today and tonight. 3. There is a risk of some wind and rainfall impacts across southeastern Cuba and Jamaica beginning Tuesday morning, and the Cayman Islands by Tuesday night. Interests there areas should monitor the progress of Grace and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 16/0900Z 17.4N 70.2W 30 KT 35 MPH 12H 16/1800Z 17.9N 72.2W 30 KT 35 MPH 24H 17/0600Z 18.6N 74.8W 30 KT 35 MPH 36H 17/1800Z 19.3N 77.4W 35 KT 40 MPH 48H 18/0600Z 20.1N 80.2W 35 KT 40 MPH 60H 18/1800Z 20.8N 83.3W 40 KT 45 MPH 72H 19/0600Z 21.6N 86.3W 45 KT 50 MPH 96H 20/0600Z 22.6N 91.8W 45 KT 50 MPH 120H 21/0600Z 23.4N 96.2W 50 KT 60 MPH $$ Forecaster Latto
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