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Tropical Storm Henri Forecast Discussion Number 16
2021-08-19 22:39:53| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Thu Aug 19 2021 000 WTNT43 KNHC 192039 TCDAT3 Tropical Storm Henri Discussion Number 16 NWS National Hurricane Center Miami FL AL082021 500 PM EDT Thu Aug 19 2021 The Air Force Hurricane Hunters investigated Henri earlier this afternoon and found maximum flight-level winds and SFMR values to support an intensity of about 55 kt and a minimum pressure of 997 mb. The storm's appearance in satellite images has been generally steady throughout the day with the low-level center located on the northern side of the convective mass, and banding features confined to the south side of the circulation. This somewhat asymmetric cloud pattern is due to about 25 kt of north-northeasterly shear. The tropical storm is now moving due westward at 9 kt. A mid-level ridge should keep Henri on a westward to west-northwestward track through tonight. On Friday, however, a trough is expected to cut off over the Ohio Valley and mid-Atlantic region of the U.S. while a ridge amplifies to the east of Henri over the western Atlantic. A combination of these features should cause Henri to turn northward on Friday and accelerate in that direction over the weekend. The exact location of the northward turn will be very critical in determining Henri's future track and how close it gets to New England. The latest GFS run has shifted eastward and the UKMET and ECMWF have shifted westward. Consequently, the net result is very little change in the consensus models and the new NHC track forecast is just a little faster than the previous one. Based on this forecast, Henri is expected to be very near southern New England on Sunday and Monday. The NOAA Gulfstream IV jet will be flying around Henri this evening to help assess the environmental conditions and gather data for the numerical models. It is hoped that these data will help the models more accurately predict the future track of the storm. Although Henri is a bit weaker now, the overall intensity forecast philosophy has not changed. The shear is expected to persist for another 12 to 24 hours, so little change in strength seems likely during that time period. However, the global models show a more conducive upper-level wind pattern over Henri shortly after that, and in fact, all of the environmental conditions appear favorable for strengthening on Friday and Saturday. Therefore, steady intensification is expected during the 24-60 h time period, and Henri is forecast to become a hurricane during that time. Henri is expected to cross the north wall of the Gulf Stream and move over much cooler SSTs on Sunday, and that should end the opportunity for strengthening and induce a steady weakening trend from days 3 to 5. Post-tropical transition is now shown to occur by the end of the forecast period when Henri is expected to be over SSTs below 20 deg C. It should be noted that as Henri gains latitude and moves near New England, the wind field is expected to expand. Therefore, 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. Watches will likely be required for portions of this area early Friday. 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 southeastern New England Sunday into Monday. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 29.7N 71.4W 55 KT 65 MPH 12H 20/0600Z 30.1N 72.6W 55 KT 65 MPH 24H 20/1800Z 31.0N 73.2W 55 KT 65 MPH 36H 21/0600Z 33.0N 72.6W 65 KT 75 MPH 48H 21/1800Z 36.0N 71.3W 75 KT 85 MPH 60H 22/0600Z 38.9N 70.3W 80 KT 90 MPH 72H 22/1800Z 40.8N 70.1W 65 KT 75 MPH 96H 23/1800Z 42.3N 69.6W 50 KT 60 MPH 120H 24/1800Z 43.6N 65.7W 40 KT 45 MPH...POST-TROPICAL $$ Forecaster Cangialosi
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