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Tropical Storm Grace Forecast Discussion Number 8
2021-08-15 10:59:18| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Aug 15 2021 000 WTNT42 KNHC 150859 TCDAT2 Tropical Storm Grace Discussion Number 8 NWS National Hurricane Center Miami FL AL072021 500 AM AST Sun Aug 15 2021 Radar data from the Leeward Islands and Puerto Rico, as well as satellite images, indicate that Grace is still not a well-organized tropical cyclone, although over the past few hours banding features have become more evident and the outflow has improved. An earlier ASCAT-C overpass showed peak winds of 30 kt associated with Grace, and the latest Dvorak intensity estimate from TAFB was 35 kt. Assuming some undersampling from the ASCAT instrument, the initial intensity estimate remains 35 kt for this advisory. Grace continues to move fairly quickly to the west-northwest, or 285/18 kt to the south of a strong mid-level ridge. Aside from the HWRF, which is a northern outlier and has not performed particularly well for Fred or Grace, and also the Canadian model, which is a southern outlier, the track guidance has come into better agreement on the future path of Grace. The cyclone is expected to slow its forward speed today, and maintain a west-northwestward motion to the south of the ridge for the next several days. The NHC track forecast is little changed from the previous one through 72 h, and then was adjusted a little to the south of the previous one thereafter. On this path, Grace would pass just south of Puerto Rico later today, cross Hispaniola tonight through Monday night, then move along the northern coast of Cuba Tuesday and Wednesday. Grace is in an environment favorable for intensification, and the NHC intensity forecast calls for modest strengthening to 45 kt before it reaches the Dominican Republic Monday morning. This portion of the forecast is in good agreement with the various consensus models. The intensity forecast becomes highly uncertain thereafter, and is dependent on how much of the Greater Antilles the cyclone interacts with. Based on the current track forecast, Grace would cross a large portion of the rugged terrain of Hispaniola, then interact with the landmass of Cuba for a couple of days. There is a decent chance that the low-level center of Grace could dissipate over Hispaniola as the system opens back into a tropical wave. However, due to the possibility of the center remaining intact after crossing that landmass, the NHC forecast calls for weakening followed by little change in strength thereafter as it moves along the Cuban coastline. The latest NHC intensity forecast beyond 24 h is little changed from the previous one, and is close to the FSU Superensemble. Key Messages: 1. Tropical storm conditions are expected over portions of the Leewards Islands this morning and the Virgin Islands and Puerto Rico today. Tropical storm conditions are expected over eastern parts of the Dominican Republic tonight and Monday. Tropical storm conditions are possible over western portions of the Dominican Republic and Haiti Monday and Monday night. 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands, Haiti and the Dominican Republic. 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 the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, Cuba, and Florida, but forecast uncertainty remains higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 15/0900Z 16.9N 64.4W 35 KT 40 MPH 12H 15/1800Z 17.5N 66.6W 40 KT 45 MPH 24H 16/0600Z 18.3N 68.9W 45 KT 50 MPH 36H 16/1800Z 18.9N 70.6W 35 KT 40 MPH...INLAND OVER HISPANIOLA 48H 17/0600Z 19.8N 72.6W 35 KT 40 MPH...INLAND OVER HISPANIOLA 60H 17/1800Z 20.9N 75.3W 35 KT 40 MPH...OVER WATER 72H 18/0600Z 22.0N 78.0W 35 KT 40 MPH...NEAR CUBA COAST 96H 19/0600Z 23.9N 82.5W 40 KT 45 MPH 120H 20/0600Z 25.6N 86.2W 45 KT 50 MPH $$ Forecaster Latto
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