[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 221435
TCDAT5

Hurricane Tammy Discussion Number  17
NWS National Hurricane Center Miami FL       AL202023
1100 AM AST Sun Oct 22 2023

Tammy is slowly pulling away from the northern Leeward Islands and
all of the hurricane warnings were dropped at the 1200 UTC
intermediate advisory.  However, there is still a trailing area of
showers and thunderstorms that will likely cause heavy rain across
portions of the Windward and Leeward Islands throughout the day.
The hurricane is exhibiting a central dense overcast pattern in
satellite images with very cold cloud tops persisting near the
center.  Both the NOAA and Air Force Hurricane Hunters are
investigating Tammy, and based on the data they have collected, the
initial intensity is nudged downward slightly to 70 kt.  Tammy
remains a fairly compact and asymmetric tropical cyclone with
hurricane-force and tropical-storm-force winds only extending up to
20 n mi and 120 n mi from the center, respectively.  The wind radii
are particularly small on the southwest side of the system.

The hurricane is moving northwestward at 8 kt on the southwestern
periphery of a subtropical ridge centered over the central Atlantic.
A turn to the north is forecast to occur tonight, followed by a
northeastward motion on Tuesday and Wednesday when a large-scale
trough over the northeast U.S. shifts eastward, eroding the ridge.
After that time, the models show the trough cutting off and merging
with Tammy, which could cause a turn to the left.  Although the
models are in better agreement than they were yesterday, there is
still a considerable amount of spread at long range due to
differences in the synoptic pattern and model predicted intensity
of Tammy.  Only small changes were made to the previous NHC track
forecast, and it continues to lie close to the HCCA guidance.

The environmental factors for Tammy appear to be mixed.  The system
is expected to remain over warm SSTs during at least the next few
days, but it will also be in an environment of moderate vertical
wind shear.  Therefore, fluctuations in strength seem likely over
the next few days.  After Tammy merges with the trough, dry air
entrainment and stronger shear should cause some weakening and
likely lead to extratropical transition in 4 to 5 days.  The NHC
intensity forecast is a little lower than the previous one, and near
the middle of the guidance envelope.


KEY MESSAGES:

1.  The heaviest rains from Tammy will continue over the Leeward
Islands through tonight.  This rainfall may produce isolated flash
and urban flooding, along with isolated mudslides in areas of higher
terrain.


FORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z 19.1N  63.0W   70 KT  80 MPH
 12H  23/0000Z 20.1N  63.6W   70 KT  80 MPH
 24H  23/1200Z 21.4N  63.9W   75 KT  85 MPH
 36H  24/0000Z 22.3N  63.5W   75 KT  85 MPH
 48H  24/1200Z 23.0N  62.9W   75 KT  85 MPH
 60H  25/0000Z 23.6N  62.0W   75 KT  85 MPH
 72H  25/1200Z 24.5N  61.0W   75 KT  85 MPH
 96H  26/1200Z 27.5N  60.5W   65 KT  75 MPH
120H  27/1200Z 30.0N  63.9W   55 KT  65 MPH...POST-TROP/EXTRATROP

$$
Forecaster Cangialosi
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