[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 180300
TCDAT3

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

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 255/08 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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