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Tropical Storm Henri Discussion Number  10
NWS National Hurricane Center Miami FL       AL082021
500 AM AST Wed Aug 18 2021
 
There's been little change in Henri's satellite appearance this 
morning. The surface center is still located just to the northwest 
of a gradually expanding central dense overcast, and there still 
appears to be an eye-like feature in the Bermuda Doppler radar 
presentation.  However, there are no indications in the GOES-16 
BD-curve enhanced infrared images of a developing warm spot. 
Subjective and objective satellite intensity estimates remain 
steady, and the initial intensity is held at 55 kt for this 
advisory.
 
Although the surrounding thermodynamic environment is ripe for 
further significant development, moderate northwesterly shear 
continues to impinge on the cyclone causing difficulty in inner-core 
convective development. Consequently, little change in strength is 
expected during the next couple days, and in fact the statistical 
SHIPS model indicates an increase in a more northerly shear 
component on Thursday.  By Saturday afternoon, however, the shear 
should decrease as the cyclone slides beneath an upper ridge axis 
off of the southeast U.S. coast.  Henri is expected to further 
intensity through the remaining period as the upper-wind flow 
becomes much more diffluent.  The official intensity forecast is 
similar to the previous advisory, and lies between the SHIPS model 
and the HFIP Corrected Consensus Approach aid after the 60-hour 
period.
 
The initial motion is estimated to be westward, or 260/7 kt.  A high 
amplitude mid- to upper-level anticyclone situated north-northeast 
of Henri should steer the cyclone toward the west during the next 36 
hours, or so.  Thereafter, the aforementioned ridge is forecast to
weaken and shift eastward in response to a mid-latitude shortwave 
trough moving over the northeastern U.S.  As a result of the change 
in the synoptic steering pattern, Henri should turn toward the 
northwest, north, and then northeast through the remaining portion 
of the forecast. With so much uncertainty or spread in the global 
and regional models beyond day 3, the best approach at this point is 
to base the NHC forecast on the better performing multi-model 
consensus guidance, which has once again shifted a bit to the left 
of the previous track forecast.  I think it's worth noting that the 
GFSv16 model is now showing a shortwave ridge building over 
eastern Canada on Sunday, which causes Henri to move more northward 
toward the New England coast.  Therefore, additional changes or 
shifts of the track beyond the 60-hour period may be required on 
subsequent advisories.

Due to the increased uncertainty in the track forecast, interests 
along the New England coast should monitor the progress of Henri.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/0900Z 30.1N  65.7W   55 KT  65 MPH
 12H  18/1800Z 29.9N  66.9W   60 KT  70 MPH
 24H  19/0600Z 29.8N  68.7W   60 KT  70 MPH
 36H  19/1800Z 29.8N  70.6W   60 KT  70 MPH
 48H  20/0600Z 30.4N  71.8W   60 KT  70 MPH
 60H  20/1800Z 31.4N  72.3W   65 KT  75 MPH
 72H  21/0600Z 33.0N  71.9W   70 KT  80 MPH
 96H  22/0600Z 37.6N  69.0W   75 KT  85 MPH
120H  23/0600Z 40.5N  66.1W   65 KT  75 MPH
 
$$
Forecaster Roberts
 
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