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Tropical Storm Grace Forecast Discussion Number 7
2021-08-15 04:58:46| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sat Aug 14 2021 000 WTNT42 KNHC 150258 TCDAT2 Tropical Storm Grace Discussion Number 7 NWS National Hurricane Center Miami FL AL072021 1100 PM AST Sat Aug 14 2021 Grace is a poorly organized tropical cyclone. Trying to pinpoint the center of the storm over the last 6-12 hours has been a challenge, even with the help of earlier NOAA aircraft, surface observations, and radar data from Guadalupe. Earlier data from the NOAA aircraft reconnaissance Twin Doppler Radar showed that the mid-level center was tilted significantly further southeast relative to the poorly-defined low-level circulation. Around 0000 UTC, multiple reporting stations in Guadalupe showed a wind shift to the west, and there were some skeletal bands seen on the nearby Guadalupe radar ahead of a convective squall propagating ahead, which is currently producing the coldest cloud tops with Grace. Given all of these data, the center of Grace is estimated to have passed by just north of Guadalupe over the last few hours. Despite the higher subjective and objective Dvorak estimates, the initial intensity was kept at 35 kt for this advisory given the lackluster wind data from the earlier NOAA aircraft reconnaissance mission. Grace appears to have reformed a bit further North tonight, and also now appears to be on a somewhat slower west-northwest heading, at 285/17 kt. There remains a large amount of spread in the guidance, even in the short-term, which is likely related to the current disorganized nature of Grace. In general, Grace is expected to maintain a west-northwestward motion and gradually slow down over the next 24-36 hours. A strong low- to mid-level ridge poleward of Grace should then maintain this west-northwest heading through the remainder of the forecast. A lot of the track uncertainty in the latter part of the forecast appears to be related to the future intensity of the storm, and both the ECMWF and GFS ensembles show a large north-to-south spread with stronger members tracking further north and weaker members tracking further south. For now, the NHC track forecast has been shifted north of the previous track, mostly related to the further north initial position, and is in close agreement with the HFIP corrected-consensus approach (HCCA) model. Grace appears to finally be slowing down a bit this evening, but will still need to slow down a bit more in order to allow the low- to mid-level centers to become better aligned. Because of this disjointed structure, only slow intensification is anticipated. The latest intensity guidance is a bit lower, and the NHC intensity guidance follows suit, with a peak intensity of 45-kt in 24 hours. Thereafter, it appears likely that Grace will have to deal with significant land interaction over Hispaniola and weakening is indicated by 48 hours. If Grace survives, it is possible some modest intensification could occur in the latter part of the forecast period. The intensity forecast remains closely tied to the track forecast, and any deviations to the north or south could allow Grace to stay stronger than indicated in this forecast. However, it also remains distinctly possible that Grace could dissipate before the end of this 5-day forecast due to the forecasted land interaction. As previously mentioned, the exact track of the center and the intensity of the system will likely not be as important as the heavy rainfall that is forecast to fall across the Leeward Islands and the Greater Antilles during the next few days. Key Messages: 1. Tropical storm conditions are expected over the Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are possible over eastern parts of the Dominican Republic Sunday night and Monday. 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 next week, 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 15/0300Z 16.8N 62.4W 35 KT 40 MPH 12H 15/1200Z 17.3N 64.6W 40 KT 45 MPH 24H 16/0000Z 18.1N 67.0W 45 KT 50 MPH...NEAR PUERTO RICO 36H 16/1200Z 18.7N 69.1W 45 KT 50 MPH...INLAND OVER HISPANIOLA 48H 17/0000Z 19.3N 70.9W 35 KT 40 MPH...INLAND OVER HISPANIOLA 60H 17/1200Z 20.2N 73.2W 35 KT 40 MPH...OVER WATER 72H 18/0000Z 21.2N 75.8W 35 KT 40 MPH 96H 19/0000Z 23.7N 81.0W 40 KT 45 MPH 120H 20/0000Z 25.6N 84.6W 45 KT 50 MPH $$ Forecaster Papin/Brown
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