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Tropical Storm HENRI (Text)


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Tropical Storm Henri Discussion Number   9...Corrected
NWS National Hurricane Center Miami FL       AL082021
1100 PM AST Tue Aug 17 2021

Corrected speed of motion in the second paragraph.

Henri's appearance on satellite imagery has remained more or less 
steady-state, featuring a small CDO with additional convective 
banding along the eastern side of the circulation. However, the 
earlier mid-level eye feature that was trying to develop on radar 
from Bermuda has recently become more ill-defined, possibly due to 
some dry-air being entrained into the inner-core of the storm. The 
latest Dvorak subjective estimates from SAB and TAFB were T3.0/45 kt 
and T3.5/55 kt, respectively. In addition, a 2336 UTC ASCAT-A pass 
had lower peak wind retrievals than what was found earlier today, 
but this instrument may not be quite able to sample the relatively 
small tropical cyclone core observed on radar. For now, the initial 
intensity will be held at 55 kt, though this estimate could be a bit 
generous given the recent scatterometer data.
 
Henri has begun a more pronounced motion to the west-southwest, and 
the latest initial motion is estimated at 255/07 kt. An amplified 
mid- to upper-tropospheric ridge located northwest of Henri is 
expected to keep the storm on a west-southwestward or westward 
heading in the short term.  However this ridge will begin to 
gradually erode as an mid- to upper-level trough propagates eastward 
to the Eastern United States. This should allow Henri to start 
gaining latitude by 48 hours, turning toward the northwest, north, 
and then northeast as the mid-level ridging redevelops southeast of 
the cyclone. There remains a large amount of spread in the guidance, 
with the stronger regional hurricane models on the left side, while 
the weaker global models remain more on the right side of the 
guidance envelope. In general though, there was another westward 
shift in the guidance suite, so the latest NHC forecast track was 
adjusted again in that direction, and is in closest agreement to the 
HCCA guidance aid.  
 
The intensity forecast in the short-term is tricky. Last night and 
this morning, Henri's deep convection was been able to propagate 
into its up-shear quadrant, in spite of light to moderate 
northwesterly shear importing fairly dry mid-latitude air from the 
north. Consequently, the storm has been able to intensify and become 
more axis-symmetrical. Over the past few hours, however, the 
convection to the northwest of the center has eroded once again on 
Bermuda radar, likely due to dry air entrainment by the 
aforementioned vertical wind shear. On the other hand, the tropical 
cyclone is currently over sea-surface temperatures above 29 C, which 
will likely allow for significant boundary layer recovery of dry 
mid-level air that is able to get into the inner core. Thus, even as 
northerly vertical wind shear increases over the next 24 hours, 
Henri is expected to maintain its intensity. After 60 hours, this 
northerly shear is expected to subside, and Henri will have an 
opportunity to intensify towards the end of the forecast. The latest 
NHC intensity forecast is quite similar to the previous one for the 
first 60 hours, but is a little stronger in the latter time periods, 
blending the reliable HCCA guidance with the more aggressive 
regional hurricane models (HWRF, COAMPS-TC).
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/0300Z 30.0N  65.1W   55 KT  65 MPH
 12H  18/1200Z 29.9N  66.0W   60 KT  70 MPH
 24H  19/0000Z 29.8N  67.7W   60 KT  70 MPH
 36H  19/1200Z 29.8N  69.5W   60 KT  70 MPH
 48H  20/0000Z 30.1N  71.1W   60 KT  70 MPH
 60H  20/1200Z 30.9N  72.0W   60 KT  70 MPH
 72H  21/0000Z 32.0N  71.9W   70 KT  80 MPH
 96H  22/0000Z 36.6N  69.4W   80 KT  90 MPH
120H  23/0000Z 40.2N  65.0W   80 KT  90 MPH
 
$$
Forecaster Papin/Brown
 
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Page last modified: Monday, 06-Dec-2021 12:09:17 UTC