[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 211451
TCDAT5

Hurricane Tammy Discussion Number  13
NWS National Hurricane Center Miami FL       AL202023
1100 AM AST Sat Oct 21 2023

The center of Tammy is now located just to the east-southeast of
Guadeloupe in the Leeward Islands.  Satellite and radar data
indicate that the core of the hurricane is quite compact and despite
its close proximity to the islands, the strong winds remain just
offshore. The NOAA and Air Force Hurricane Hunters have been
investigating Tammy this morning, and a combination of the data they
have collected indicate that the hurricane has strengthened a
little. The initial intensity is nudged upward to 75 kt based on
maximum 700 mb flight-level winds of 86 kt and a peak SFMR wind of
74 kt. However, as noted above, the hurricane-force winds are
occurring over a small region very near the center.

The hurricane has turned to the right over the past several hours,
and the initial motion is now northwestward, 325 degrees, at 7 kt.
A continued northwestward motion is expected during the next 12-24
hours, taking the core of Tammy over or just east of the Leeward
Islands during that time.  After the hurricane clears the islands,
a turn to the north seems likely when the system moves in the flow
between a ridge over the central Atlantic and a large trough over
the western Atlantic.  There is a significant amount of spread in
the models in the days 3-5 time frame, which seems to be associated
with model differences in the magnitude of the trough and vertical
depth/intensity of Tammy.  The NHC track forecast is a little to
the right and slower than the previous one based on the initial
position/motion and latest model guidance.  However, given that the
spread in the model positions of Tammy are about 900 miles at day 5,
confidence is low in Tammy's long range track.

Tammy will likely fluctuate in strength over the next few days
while it remains in mixed environmental conditions of very warm
SSTs and moderate wind shear.  The NHC intensity forecast is a
touch higher than the previous one and generally near the IVCN and
HCCA models. Gradual weakening seems likely by the middle of next
week when it moves into a region of stronger shear, but the
intensity forecast at that range is of low confidence since its
future strength will likely be correlated to the track.


KEY MESSAGES:

1.  Hurricane and tropical storm conditions are expected in portions
of the Leeward and northern Windward Islands through early Sunday.

2.  Heavy rains from Tammy will affect the Lesser Antilles north of
Martinique today, spreading across the British and U.S. Virgin
Islands and possibly eastern Puerto Rico on Sunday. This rainfall
may produce isolated flash and urban flooding, along with isolated
mudslides in areas of higher terrain.

3.  A storm surge could produce coastal flooding in areas of onshore
winds as the center of Tammy moves near or over the Leeward Islands.
Near the coast, the surge will be accompanied by large and dangerous
waves.


FORECAST POSITIONS AND MAX WINDS

INIT  21/1500Z 16.0N  60.7W   75 KT  85 MPH
 12H  22/0000Z 17.0N  61.6W   75 KT  85 MPH
 24H  22/1200Z 18.6N  62.5W   80 KT  90 MPH
 36H  23/0000Z 20.0N  63.3W   80 KT  90 MPH
 48H  23/1200Z 21.3N  63.8W   80 KT  90 MPH
 60H  24/0000Z 22.3N  63.8W   80 KT  90 MPH
 72H  24/1200Z 23.2N  63.5W   80 KT  90 MPH
 96H  25/1200Z 24.9N  62.7W   75 KT  85 MPH
120H  26/1200Z 27.3N  62.0W   65 KT  75 MPH

$$
Forecaster Cangialosi
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