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