[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 201459
TCDAT5

Hurricane Tammy Discussion Number   9
NWS National Hurricane Center Miami FL       AL202023
1100 AM AST Fri Oct 20 2023

Recent data from both NOAA and Air Force Reserve Hurricane Hunter
aircraft and radar imagery from Barbados shows that the inner core
structure of the storm has become much better organized this
morning. The radar imagery has depicted strong convection that has
quickly evolved into a curved band, and more recently a closed eye.
The aircraft data indicates that there is still some tilt to the
circulation, but the most recent center fixes indicate that the
pressure has fallen quickly to around 992 mb this morning.  The
plane has reported flight-level winds of 71 kt at 700 mb, and SFMR
values of 65-70 kt.  Based on these data, the initial intensity has
been increased to 65 kt for this advisory, making Tammy a hurricane.

Tammy is moving slowly west-northwestward at about 6 kt. The storm
should turn northwestward later today as an amplified trough moving
off the east coast of the U.S. and into the western Atlantic erodes
the western portion of a subtropical ridge over the central
Atlantic. This motion is expected to bring the center of Tammy near
or over portions of the Leeward Islands tonight and Saturday. The
latest track prediction is similar to the previous advisory and
lies closest to the HFIP corrected consensus during the first
36-48 hours.  After Tammy moves north of the Leeward Islands, the
storm is forecast to turn northward and then begin to recurve in
the mid-latitude westerlies by days 4 and 5. There is increasing
along-track spread in the guidance after 72 hours due to how fast
Tammy accelerates northeastward.  Much of the guidance has been
trending slower, and the official forecast has followed suit.

The environment consisting of warm SSTs and light to moderate
shear favors some additional strengthening during the next couple of
days. The updated NHC intensity forecast calls for strengthening
during the next 36-48 hours while Tammy approaches the Leeward
Islands. Once Tammy is north of the Leeward Islands, increasing
vertical wind shear is likely to limit additional intensification.
Near the end of the period, Tammy is likely to begin its
extratropical transition, but it is still shown to be a hurricane
at day 5.


KEY MESSAGES:

1.  Hurricane conditions are expected in portions of the Leeward
Islands by late tonight or Saturday morning. Tropical storm
conditions are expected to begin in portions of the Lesser Antilles
within the warning area later today.

2.  Heavy rains from Tammy will begin to affect the Leeward and
northern Windward Islands later today and into Saturday morning,
spreading into the British and U.S. Virgin Islands and Puerto Rico
as early as 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  20/1500Z 14.1N  58.6W   65 KT  75 MPH
 12H  21/0000Z 15.1N  59.8W   70 KT  80 MPH
 24H  21/1200Z 16.4N  60.9W   75 KT  85 MPH
 36H  22/0000Z 17.9N  61.9W   75 KT  85 MPH
 48H  22/1200Z 19.3N  62.7W   80 KT  90 MPH
 60H  23/0000Z 20.6N  63.3W   80 KT  90 MPH
 72H  23/1200Z 22.0N  63.3W   80 KT  90 MPH
 96H  24/1200Z 24.4N  61.4W   80 KT  90 MPH
120H  25/1200Z 28.2N  57.7W   75 KT  85 MPH

$$
Forecaster Brown
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