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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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