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Tropical Storm Henri Forecast Discussion Number 20
2021-08-20 22:42:48| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Fri Aug 20 2021 000 WTNT43 KNHC 202042 TCDAT3 Tropical Storm Henri Discussion Number 20 NWS National Hurricane Center Miami FL AL082021 500 PM EDT Fri Aug 20 2021 Henri appears to be slowly gaining strength. Deep convection has been increasing during the past few hours and the upper-level outflow continues to become better established on the east side of the circulation. However, the low-level center is still located near the northwestern edge of the deep convection due to ongoing northerly wind shear. An ASCAT-B pass from a few hours ago showed peak winds of around 55 kt, with the strongest winds on the southeast side of the circulation. Based on this data, the initial intensity is set at 60 kt. An Air Force Reserve Hurricane Hunter aircraft is scheduled to investigate Henri again this evening and the data the plane collects will be very helpful in estimating the storm's strength and structure. The tropical storm appears to be making the advertised turn to the right, with the latest initial motion estimated to be 335/6 kt. A trough over the central Appalachians is expected to cut off tonight, and the combination of that feature and a building ridge to the east/northeast of Henri should cause the storm to accelerate to the north on Saturday. The latest model runs have generally trended to the left again and are a bit faster, with most showing landfall in about 48 hours. Most of the models now show a slight left turn before landfall as Henri gets caught in the circulation of the aforementioned trough. The latest GFS and ECMWF ensemble members continue to span a region from near New York City to Cape Cod for the landfall point. Based on the latest consensus aids, the official track forecast has again been nudged to the west of the previous one and shows landfall occurring on Sunday. After day 3, Henri is forecast to eject northeastward. The ongoing northerly shear is expected to decrease tonight, and the global models show a favorable upper-level wind pattern developing over the storm through the weekend. These more conducive atmospheric conditions combined with the warm Gulf Stream waters should allow Henri to strengthen to a hurricane tonight or early Saturday, with additional intensification expected into Saturday night. By early Sunday, however, Henri is forecast to cross the north wall of the Gulf Stream, and that should end its opportunity for strengthening. Although steady to rapid weakening is forecast while Henri moves over cooler waters south of Long Island and New England, it will likely still be at or very near hurricane intensity when it reaches the coast. Post-tropical transition is forecast to occur by day 3 and the system should dissipate in 4 or 5 days. Users are reminded to not focus on the exact forecast of the center of Henri since impacts will extend far from the center. Key Messages: 1. Dangerous storm surge inundation is expected to begin Sunday in portions of Long Island, Connecticut, Rhode Island, and southeastern Massachusetts, where a Storm Surge Warning has been issued. Dangerous storm surge is possible beginning Sunday in western portions of Long Island and Connecticut in the Storm Surge Watch area. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are expected to begin Sunday in portions of Long Island and Connecticut, where a Hurricane Warning has been issued. Hurricane conditions are possible Sunday across portions of Rhode Island and southeastern Massachusetts. 3. Heavy rainfall may lead to considerable flash, urban, and small stream flooding, along with the potential for widespread minor and isolated moderate river flooding, over portions of Long Island and New England Sunday into Monday. 4. Swells from Henri will continue to affect much of the east coast of the U.S. through the weekend. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 31.2N 73.9W 60 KT 70 MPH 12H 21/0600Z 32.8N 73.3W 65 KT 75 MPH 24H 21/1800Z 35.8N 72.3W 75 KT 85 MPH 36H 22/0600Z 38.9N 71.9W 75 KT 85 MPH 48H 22/1800Z 40.8N 72.4W 65 KT 75 MPH 60H 23/0600Z 42.0N 72.8W 45 KT 50 MPH...INLAND 72H 23/1800Z 42.9N 72.6W 30 KT 35 MPH...POST-TROPICAL 96H 24/1800Z 43.8N 68.9W 25 KT 30 MPH...POST-TROPICAL 120H 25/1800Z...DISSIPATED $$ Forecaster Cangialosi
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