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Tropical Storm Henri Forecast Discussion Number 14
2021-08-19 10:54:42| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Aug 19 2021 000 WTNT43 KNHC 190854 TCDAT3 Tropical Storm Henri Discussion Number 14 NWS National Hurricane Center Miami FL AL082021 500 AM AST Thu Aug 19 2021 Compared to microwave images from earlier on Wednesday, GMI microwave data from last evening revealed that Henri's structure had deteriorated somewhat, with convection on the west side of the mid-level eye having mostly dissipated. This degradation is likely the result of strong deep-layer shear and dry air in the mid levels. Satellite intensity estimates have either remained steady or decreased a bit, and Henri's initial intensity is therefore held at 60 kt. This value is supported by two evening scatterometer passes, which had peak winds of 52 kt and 57 kt. The GMI and ASCAT data revealed that the center is slightly farther south than previously estimated, and Henri has been moving south of due west, or 260/8 kt. Water vapor imagery shows a shortwave trough moving across the Great Lakes toward the Ohio Valley, and global models are in agreement that this trough will cut off over the central Appalachians in about 48 hours. As a result, the ridge currently steering Henri westward is expected to relocate over the northern Gulf coast, with the cut-off low causing Henri to accelerate northward between Bermuda and the east coast of the United States by late Friday through Sunday. Then, mid-level ridging over Quebec is likely to cause Henri to slow down considerably in the vicinity of southeastern New England or the adjacent offshore waters by Monday. The latest suite of deterministic track models have much less spread compared to on Wednesday, with fairly good agreement on the scenario described above. However, both the GEFS and ECMWF ensembles continue to show a wider assortment of solutions, with stronger storms tending to move closer to the U.S. coast and weaker storms moving farther offshore. With the tight clustering of the current guidance, the trajectory of the new NHC track forecast was not changed much from the previous iteration, although it is a little faster during the time that Henri accelerates to the north. The biggest point here is that it's still too soon to know exactly how close Henri's center will get to the coast of New England. The north-northeasterly shear affecting Henri is not expected to abate for another 24-36 hours. Once the shear does decrease, however, warm waters should foster strengthening, up until Sunday when Henri is expected to move north of the Gulf Stream. An increase in southerly shear and Henri's slow motion over the colder waters off New England should then cause weakening on days 4 and 5. The NHC intensity forecast is a little below the HCCA and IVCN consensus aids, which are being influenced by the seemingly over-aggressive HWRF and COAMPS-TC models, and this new forecast is very similar to the previous prediction. Key Messages: 1. Henri is forecast to be near the northeast coast of the U.S. late this weekend and early next week, and the risks of storm surge, wind, and rain impacts in portions of the northeastern U.S. and Atlantic Canada remains a distinct possibility. Interests in these areas should closely follow the progress of Henri and check for updates to the forecast. 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. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 29.5N 69.5W 60 KT 70 MPH 12H 19/1800Z 29.5N 70.9W 60 KT 70 MPH 24H 20/0600Z 29.9N 72.4W 60 KT 70 MPH 36H 20/1800Z 31.0N 73.0W 65 KT 75 MPH 48H 21/0600Z 33.0N 72.5W 75 KT 85 MPH 60H 21/1800Z 35.7N 71.4W 80 KT 90 MPH 72H 22/0600Z 38.2N 70.2W 80 KT 90 MPH 96H 23/0600Z 41.1N 69.3W 60 KT 70 MPH 120H 24/0600Z 42.0N 67.5W 50 KT 60 MPH $$ Forecaster Berg
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