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Tropical Storm Henri Forecast Discussion Number 18
2021-08-20 10:59:34| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Fri Aug 20 2021 000 WTNT43 KNHC 200859 TCDAT3 Tropical Storm Henri Discussion Number 18 NWS National Hurricane Center Miami FL AL082021 500 AM EDT Fri Aug 20 2021 Henri's low-level center has been peaking out from under the north side of the deep convective mass, resulting from continued 20-25 kt of northerly shear. Maximum winds are still estimated to be 55 kt based on T3.5 Dvorak estimates from TAFB and SAB. Henri still has a motion toward the west-northwest, or 300/8 kt, but it is about ready to make the sharp right turn that we've been expecting. A shortwave trough currently over the central Appalachians is forecast to close off by Saturday, with Henri accelerating northward on the east side of this feature through the weekend. Some global models show Henri merging with the mid-/upper-level low as the cyclone approaches southern New England, which induces a slight bend of Henri's forecast track to the left. Nearly all track models now show Henri's center reaching the coast of southern New England, and the new NHC track forecast has been nudged westward in the direction of HCCA and the other consensus aids. After day 3, Henri is forecast to eject northeastward across the Gulf of Maine toward Atlantic Canada. The strong shear affecting Henri is forecast to begin weakening later today, and SHIPS diagnostics indicate that it could drop to less than 10 kt in about 36 hours. In addition, Henri will be traversing very warm waters for the next 48 hours before it crosses the north wall of the Gulf Stream. Therefore, strengthening is anticipated for the next 2 days, with Henri likely to become a hurricane by Saturday. After 48 hours, Henri's expected slower motion over the colder water south of New England should induce quick weakening, but it may not be quick enough to keep Henri from reaching the coast as a hurricane. Faster weakening is anticipated after Henri's center moves over land, and simulated satellite imagery from the GFS and ECMWF models suggest that deep convection could dissipate by day 4, making Henri a post-tropical low at that time. Global models suggest that Henri may dissipate by day 5, but for the time being a day 5 point is being kept for continuity. Based on the new forecast, tropical-storm-force wind radii will be approaching the coast of southern New England in about 48 hours. Given the still-present uncertainties in Henri's future track and intensity and the hazards that the storm may cause, storm surge and hurricane watches are now being issued for portions of Long Island, Connecticut, Rhode Island, and southeastern Massachusetts. Users are reminded to not focus on the exact forecast points as impacts will extend far from the center. Key Messages: 1. Henri is forecast to be near the northeast coast of the U.S. on Sunday and Monday, and the risks of storm surge, wind, and rain impacts in portions of southern New England and eastern Long Island are increasing. Hurricane and storm surge watches are now in effect for portions of Long Island, Connecticut, Rhode Island, and southeastern Massachusetts. Additional watches or warnings may be required later today. 2. Swells from Henri will begin to reach much of the east coast of the U.S. and Atlantic Canada by the end of the week and continue through the weekend. These swells could cause life-threatening surf and rip currents. 3. Heavy rainfall may lead to flash, urban, and small stream flooding over portions of southern New England Sunday into Monday. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 30.2N 73.1W 55 KT 65 MPH 12H 20/1800Z 31.0N 73.5W 55 KT 65 MPH 24H 21/0600Z 33.0N 72.9W 60 KT 70 MPH 36H 21/1800Z 36.1N 71.8W 70 KT 80 MPH 48H 22/0600Z 39.1N 71.1W 75 KT 85 MPH 60H 22/1800Z 41.1N 71.3W 65 KT 75 MPH 72H 23/0600Z 42.2N 71.6W 50 KT 60 MPH...INLAND 96H 24/0600Z 43.5N 70.0W 35 KT 40 MPH...POST-TROPICAL 120H 25/0600Z 45.3N 63.0W 30 KT 35 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg
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