[Tropical] Tropical Cyclone Discussion

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Tropical Storm Henri Discussion Number  19
NWS National Hurricane Center Miami FL       AL082021
1100 AM EDT Fri Aug 20 2021

The Air Force Reserve Hurricane Hunters investigated Henri this
morning and based on the flight-level, SFMR, and dropsonde data, the
initial intensity is still around 55 kt.  The minimum pressure has
also been relatively steady.  The low-level center of Henri is
located close to the northwestern edge of the main area of deep
convection, and the vortex is still not vertically aligned due to
20-25 kt of northerly vertical wind shear.  Although the intensity
of Henri has not changed much during the past couple of days,
satellite images show an improving cloud pattern with well-
established outflow in the eastern semicircle, which could indicate
that Henri is poised to strengthen.

There is some uncertainty in the exact position of Henri given the
recent aircraft fixes and the tilt of the vortex, but the initial
motion appears to be northwestward at 6 kt.  A trough over the
central Appalachians is expected to cut off tonight, and the
combination of that feature and a building ridge to the
east/northeast of Henri should cause the storm to turn northward
tonight and accelerate in that direction on Saturday. Some of the
models show a slight bend to the west around the time Henri is
forecast to make landfall on Sunday, and there remains a fair amount
spread in the guidance in where the center of Henri will come
ashore.  The latest GFS and ECMWF ensemble members span a region
from near New York City to Cape Cod for the landfall point.  Based
on the latest consensus aids and initial position, the official
track forecast is nudged to the west of the previous one and shows
landfall occurring by late Sunday.  After day 3, Henri is forecast
to eject northeastward across the Gulf of Maine.

The ongoing northerly shear is expected to decrease later today, and
the global models show a favorable upper-level wind pattern
developing over the storm later today through the weekend.  These
more conducive atmospheric conditions combined with the warm Gulf
Stream waters should allow Henri to strengthen to a hurricane
tonight or early Saturday with additional intensification expected
into Saturday night.  By early Sunday, however, Henri is forecast to
cross the north wall of the Gulf Stream, and that should end its
opportunity for strengthening. Although steady to rapid weakening is
forecast while Henri moves over cooler waters south of New England,
it will likely still be at or very near hurricane intensity when it
reaches the coast. Post-tropical transition is forecast to occur in
3 to 4 days, and the new forecast shows Henri dissipated by day 5,
in agreement with most of the global models.

Users are reminded to not focus on the exact forecast of the
center of Henri since impacts will extend far from the center.

Key Messages:

1. Dangerous storm surge inundation and hurricane conditions are
possible beginning Sunday in portions of Long Island, Connecticut,
Rhode Island, and southeastern Massachusetts, where a Storm Surge
Watch and Hurricane Watch are in effect. Residents in these areas
should follow any advice given by local officials.

2. Heavy rainfall may lead to flash, urban, and small stream
flooding, along with the potential for widespread minor and isolated
moderate river flooding, over portions of southern New England
Sunday into Monday.

3. 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  20/1500Z 30.4N  73.7W   55 KT  65 MPH
 12H  21/0000Z 31.5N  73.7W   60 KT  70 MPH
 24H  21/1200Z 34.1N  72.7W   70 KT  80 MPH
 36H  22/0000Z 37.3N  71.8W   75 KT  85 MPH
 48H  22/1200Z 39.8N  71.7W   65 KT  75 MPH
 60H  23/0000Z 41.4N  71.8W   60 KT  70 MPH...INLAND
 72H  23/1200Z 42.4N  71.8W   40 KT  45 MPH...INLAND
 96H  24/1200Z 43.7N  69.4W   35 KT  40 MPH...POST-TROPICAL
120H  25/1200Z...DISSIPATED

$$
Forecaster Cangialosi
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