[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 190854
TCDAT5

Tropical Storm Tammy Discussion Number   3
NWS National Hurricane Center Miami FL       AL202023
500 AM AST Thu Oct 19 2023

Tammy has not changed much overnight.  Satellite images show a
persistent area of deep convection, but the pattern remains ragged
with little evidence of banding features.  The low-level center has
occasionally been exposed near the western edge of the convection
due to westerly vertical wind shear.  The initial intensity is held
at 35 kt, in agreement with the Dvorak classification from TAFB.
The NOAA Hurricane Hunters will be investigating the storm later
this morning, and the data they collect will provide a better
assessment of Tammy's intensity, center position, and overall
structure.

The storm is moving generally westward at about 15 kt.  Tammy is
expected to turn west-northwestward later today and likely
northwestward on Friday as it moves along the southwestern periphery
of a subtropical ridge over the central Atlantic.  This should take
Tammy near or over the Leeward Islands Friday and Friday night.
After that time, a gradual turn to the north and then the northeast
is predicted as Tammy moves in the flow between the ridge and an
approaching mid- to upper-level trough over the western Atlantic.
The NHC track forecast is similar to the previous one in the short
term, but is slower at days 4 and 5 based on the new model guidance.
The GFS and ECMWF models have come into better agreement this
cycle, and both lie a little to the right of the NHC forecast
during from 12 to 72 hours.

Tammy will be moving over very warm 30 C SSTs as it nears the
Leeward Islands, which should allow the storm to strengthen despite
continued moderate wind shear.  The storm will likely be near
hurricane strength when it moves near the Leeward Islands.
Additional strengthening is expected when the system recurves over
the central Atlantic while it remains over warm water and also gains
baroclinic support from the aforementioned trough. The NHC
intensity forecast is largely an update of the previous one and is
in line with the HCCA and IVCN consensus guidance.


KEY MESSAGES:

1.  Tropical storm conditions are possible across portions of the
Lesser Antilles beginning on Friday, where tropical storm
watches are currently in effect. Additional watches and warnings
will likely be required later today.

2.  Heavy rains from Tammy will begin to affect the northern
Windward and Leeward Islands on Friday, spreading into the British
and U.S. Virgin Islands and Puerto Rico over the weekend.  This
rainfall may produce isolated flash and urban flooding, along with
isolated mudslides in areas of higher terrain.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 13.5N  54.8W   35 KT  40 MPH
 12H  19/1800Z 13.9N  56.4W   40 KT  45 MPH
 24H  20/0600Z 14.5N  58.2W   50 KT  60 MPH
 36H  20/1800Z 15.5N  60.0W   50 KT  60 MPH
 48H  21/0600Z 16.8N  61.3W   55 KT  65 MPH
 60H  21/1800Z 18.4N  62.4W   55 KT  65 MPH
 72H  22/0600Z 19.9N  63.1W   60 KT  70 MPH
 96H  23/0600Z 23.3N  63.0W   70 KT  80 MPH
120H  24/0600Z 27.2N  59.5W   70 KT  80 MPH

$$
Forecaster Cangialosi
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