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Tropical Storm Henri Discussion Number  11
NWS National Hurricane Center Miami FL       AL082021
1100 AM AST Wed Aug 18 2021
 
Deep convection has been increasing during the past several hours in 
association with Henri, and the cloud pattern mostly consists of a 
central dense overcast with some fragmented bands around it.  The 
Dvorak estimates are steady at 3.5/55 kt from TAFB and SAB, and 
therefore, the initial intensity is held at that value.  However, 
the latest automated Dvorak values from CIMSS at the University of 
Wisconsin are higher, so it is possible that Henri could be slightly 
stronger.  An SSMIS pass from 1124 UTC showed some southward tilt 
of the vortex with height and revealed a small mid-level eye 
feature.
 
Henri continues to move just south of due west at 7 kt.  The storm 
is expected to move westward during the next day or so as it moves 
in the flow on the south or southeast side of a mid-level ridge.  
This ridge is expected to slide eastward by Friday as a mid- to 
upper-level low develops over the Ohio Valley and northeastern U.S. 
In response, Henri is expected to turn northward or north- 
northeastward on Friday and continue in that direction through the 
weekend.  One complicating factor is that several of the models show 
a ridge building over the northwestern Atlantic and Atlantic Canada, 
which could cause the storm to stay on a more northward track closer 
to New England than currently forecast.  The spread in the models 
for Henri's future track is quite large, with some guidance as far 
west as Long Island and as far east as 62W longitude.  Overall, the 
models continue to walk left each cycle, and the NHC track forecast 
has again been shifted in that direction.  NOAA Gulfstream IV 
missions and special weather balloon soundings have been scheduled, 
and it is hoped that this additional data will help the models 
handle the evolving steering pattern.  Given the uncertainty in the 
longer range track forecasts, users should be prepared for 
additional adjustments to the NHC track forecast in future forecast 
cycles. 

The storm is currently in an environment of about 15-20 kt of 
northerly wind shear according to the SHIPS model and analyses from 
CIMSS at the University of Wisconsin.  Since the shear is expected 
to persist or become a little stronger during the next couple of 
days, little change in strength is predicted during that time 
period.  After that time, the shear is expected to lessen and that 
should allow Henri to strengthen to a hurricane by the weekend. Some 
weakening is shown by the end of the period when Henri is forecast 
to be moving over cooler waters.  The NHC intensity forecast is 
largely an update of the previous one and is just a little lower 
than the consensus aids.

Key Messages:
 
1. Swells from Henri could 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.  

2. Uncertainty in the track forecast for Henri this weekend and 
early next week is larger than usual, and there is some risk of 
direct impacts from Henri in portions of the northeastern U.S. and 
Atlantic Canada during that time. Interests in these areas should 
follow updates to the forecast through the next several days.

FORECAST POSITIONS AND MAX WINDS
 
INIT  18/1500Z 30.0N  66.6W   55 KT  65 MPH
 12H  19/0000Z 29.9N  67.9W   60 KT  70 MPH
 24H  19/1200Z 29.8N  69.8W   60 KT  70 MPH
 36H  20/0000Z 30.1N  71.4W   60 KT  70 MPH
 48H  20/1200Z 30.7N  72.4W   60 KT  70 MPH
 60H  21/0000Z 32.1N  72.5W   65 KT  75 MPH
 72H  21/1200Z 34.2N  71.5W   75 KT  85 MPH
 96H  22/1200Z 38.0N  69.6W   75 KT  85 MPH
120H  23/1200Z 41.0N  67.0W   65 KT  75 MPH
 
$$
Forecaster Cangialosi
 
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