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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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