[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 191501
TCDAT5

Tropical Storm Tammy Discussion Number   5
NWS National Hurricane Center Miami FL       AL202023
1100 AM AST Thu Oct 19 2023

Tammy is a sheared tropical cyclone with the center located on the
northwestern edge of the main convective mass.  Despite this
structure, the storm has strengthened since yesterday. A NOAA
Hurricane Hunter aircraft that investigated Tammy this morning found
peak 700-mb flight-level winds of 49 kt and peak SFMR winds of 50
kt. Based on these data, the intensity was increased to 50 kt on a
Special advisory that was issued at 1200 UTC.  The plane did not
find any stronger winds since that time, and the initial intensity
remains 50 kt for this advisory.  An Air Force Reserve
reconnaissance aircraft is scheduled to investigate Tammy this
afternoon.

Tammy is moving west-northwestward or 285 degrees at 13 kt. The
storm is forecast to reach the southwestern periphery of a
subtropical ridge over the central Atlantic by Friday, and this
should cause Tammy to slow down and turn northwestward during the
next 24 to 36 hours. This motion is forecast to bring the center of
the storm near or over portions of the Leeward Islands late Friday
and Saturday.  After Tammy moves north of the Leeward Islands, the
storm is expected to turn northward around the western side of the
ridge.  There is still significant spread on exactly when the
northwestward turn occurs with the GFS and ECMWF along the right
side of the guidance envelope, with some of the regional hurricane
models along the left side.  The NHC track is near the middle of
the guidance envelope closest to the HFIP corrected consensus
model.

Tammy will be moving over warm SSTS as it nears the Leeward Islands,
which should allow for gradual strengthening despite the presence
of light to moderate wind shear.  Given the current sheared
structure, the NHC intensity forecast is a bit more conservative
during the first day or two than much of the intensity guidance.
Despite that, the updated NHC forecast calls for Tammy to be at or
near hurricane strength when it moves near or over the Leeward
Islands.   Additional strengthening is expected after Tammy moves
north of the Leeward Islands and recurves over the central
Atlantic.


KEY MESSAGES:

1.  Tropical storm conditions are expected to begin in portions
of the Lesser Antilles on Friday, where a tropical storm warning is
in effect.  Tropical storm and hurricane conditions are possible
elsewhere in the Leeward Islands where hurricane and 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  19/1500Z 13.6N  55.7W   50 KT  60 MPH
 12H  20/0000Z 14.0N  57.4W   55 KT  65 MPH
 24H  20/1200Z 14.8N  59.2W   60 KT  70 MPH
 36H  21/0000Z 16.0N  60.9W   60 KT  70 MPH
 48H  21/1200Z 17.5N  62.1W   65 KT  75 MPH
 60H  22/0000Z 19.0N  62.8W   70 KT  80 MPH
 72H  22/1200Z 20.5N  63.3W   70 KT  80 MPH
 96H  23/1200Z 24.0N  62.8W   75 KT  85 MPH
120H  24/1200Z 28.4N  57.9W   75 KT  85 MPH

$$
Forecaster Brown
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