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Tropical Storm Grace Forecast Discussion Number 4
2021-08-14 10:40:14| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Aug 14 2021 000 WTNT42 KNHC 140840 TCDAT2 Tropical Storm Grace Discussion Number 4 NWS National Hurricane Center Miami FL AL072021 500 AM AST Sat Aug 14 2021 The cyclone has become a little better organized overnight, with an area of concentrated convection persisting over the center for the past several hours. The system remains compact, with the convective canopy about 100 n mi in diameter, while earlier ASCAT data revealed winds greater than 25 kt extended only about 30 n mi from its center, with peak winds of 27 kt. Despite the weaker maximum winds sampled earlier this evening, the increase in organization of such a small system has likely produced a notable increase in the surface winds near the center. In addition the latest Dvorak intensity estimates from TAFB, SAB, and the UW-CIMSS ADT and SATCON suggest that the intensity is now somewhere between 30 and 45 kt. Based on a conservative blend of these values, it is estimated that the Tropical Depression Seven has strengthened into 35-kt Tropical Storm Grace. Grace continues to move quickly westward, with an initial motion of 280/19 kt. A strong mid-level ridge to the north of the storm should continue to steer it westward for the next few days. Beyond that time, the forecast models begin to diverge in their track solutions, as they vary in the strength of the ridge. Overall, the models have trended toward a stronger, more persistent ridge to the north of the cyclone for the latter half of the forecast period, and as such the model solutions are generally showing a track that lies a bit south of the previous runs for those time periods. The official NHC forecast track is little changed through 72 h and lies near the middle of the tightly clustered track guidance. Beyond 72 h, the forecast track has been shifted slightly to the south, but still lies to north of the consensus model tracks. The earlier bout of wind shear that had entrained surrounding dry air into Grace appears to have abated, at least in the short term. And, global models indicate that the shear will remain low for about the next 24-36 h while the cyclone moves over increasing SSTs. Therefore, it is reasonable to assume that strengthening should occur over the next day or so. Thereafter, the intensity forecast becomes somewhat complicated as the system is forecast to interact with an upper-level trough while possibly crossing the Greater Antilles at the same time. Slight weakening is indicated in the NHC intensity forecast as the system crosses the northern portion of the Dominican Republic early next week. Thereafter, moderate to strong northerly shear is forecast to impact Grace as the system moves into the flow on the western side of an upper-level trough over the western Atlantic. This shear should prevent any further strengthening through the end of the forecast period. The intensity forecast is a little lower than a blend of the NOAA HCCA and IVCN consensus, as there is likely a high bias from the HWRF in these solutions. The latter portion of the NHC intensity forecast is of lower-than-normal confidence due to the potential for a longer amount of time over the rugged terrain of the Greater Antilles than indicated, and also due to the fragile nature of the small cyclone and the possible effects of the stronger shear later in the forecast period. Key Messages: 1. Tropical storm conditions are expected in portions of the Leeward Islands by tonight or early Sunday, and the U.S. Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are also possible over the British Virgin Islands on Sunday. The risk of strong winds will then spread westward to 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, 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 Haiti, the Turks and Caicos Islands, the southeastern Bahamas, Cuba, and Florida next week, and interests in those areas should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 15.8N 55.6W 35 KT 40 MPH 12H 14/1800Z 16.2N 58.5W 35 KT 40 MPH 24H 15/0600Z 16.9N 61.8W 40 KT 45 MPH 36H 15/1800Z 17.7N 64.8W 45 KT 50 MPH 48H 16/0600Z 18.3N 67.3W 45 KT 50 MPH 60H 16/1800Z 18.9N 69.4W 40 KT 45 MPH...INLAND 72H 17/0600Z 19.8N 71.5W 40 KT 45 MPH...INLAND 96H 18/0600Z 21.9N 75.6W 40 KT 45 MPH...OVER WATER 120H 19/0600Z 24.5N 79.7W 40 KT 45 MPH $$ Forecaster Latto
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