Hurricane Tammy Discussion Number  10
NWS National Hurricane Center Miami FL       AL202023
500 PM AST Fri Oct 20 2023

Moderate shear that is likely undercutting the higher-level outflow
layer has disrupted the inner-core development seen earlier today.  
The eye that became apparent in radar imagery from Barbados this 
morning has degraded since that time, but there is still a large 
curved band that wraps around the eastern and southern portions of 
the circulation.  The Air Force aircraft that investigated Tammy 
through early afternoon measured SFMR winds of around 65 kt during 
its final pass through the center, and it reported that the minimum 
pressure had fallen to around 991 mb.  Although there has been the 
recent degradation of the inner core, the initial intensity remains 
65 kt, and is based on the earlier aircraft data and more recent 
subjective satellite estimates from TAFB and SAB. The next 
reconnaissance mission into Tammy is scheduled for this evening.

Very recent radar imagery from Barbados suggest that Tammy may 
finally be making its anticipated northwestward turn, however the 
long-term motion is still west-northwestward about 6 kt.  A more 
pronounced northwestward motion should begin very soon as a 
trough moving off the east coast of the United States erodes the 
western portion of a subtropical ridge over the central Atlantic.  
The expected northwestward motion should bring the center of Tammy 
near or over portions of the Leeward Islands later tonight and 
Saturday.  While the numerical model guidance is in relatively good 
agreement during the first 48-72 hours of the forecast period, the 
spread has greatly increased after that time.  The GFS shows a 
stronger and more vertically deep cyclone recurving over 
west-central Atlantic after day 3, while the latest UKMET and ECMWF 
models depict a weaker cyclone that is left behind by an 
eastward-moving trough over the western Atlantic.  The NHC forecast 
continues to predict recurvature, although the latter portion has 
been adjusted slower once again.  

The environment ahead of Tammy is not predicted to change much 
during the next day or so.  Warm waters and moderate shear are 
expected to allow for some modest strengthening during the next 
couple of days, and the NHC intensity forecast during that time is 
similar to the previous advisory.  After Tammy moves north of the 
Leeward Islands, increasing vertical wind shear is likely to 
initiate weakening as Tammy moves northward.  The NHC intensity 
forecast is close to the IVCN intensity aid.  Due to the 
differences in how fast Tammy recurves, there is more uncertainty 
than normal on when Tammy will begin extratropical transition.  
The current NHC forecast maintains Tammy as a hurricane through day 
5, but if it trends toward the faster side of the guidance it could 
be post-tropical by that time. 

KEY MESSAGES:

1.  Hurricane conditions are expected in portions of the Leeward
Islands by late tonight through Saturday. Tropical storm
conditions are expected to begin in portions of the Lesser Antilles
within the warning area this evening or overnight.

2.  Heavy rains from Tammy will begin to affect the Leeward and 
northern Windward Islands tonight and Saturday 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  20/2100Z 14.3N  59.2W   65 KT  75 MPH
12H  21/0600Z 15.3N  60.1W   65 KT  75 MPH
24H  21/1800Z 16.7N  61.2W   70 KT  80 MPH
36H  22/0600Z 18.1N  62.1W   75 KT  85 MPH
48H  22/1800Z 19.5N  63.0W   80 KT  90 MPH
60H  23/0600Z 20.8N  63.4W   80 KT  90 MPH
72H  23/1800Z 22.0N  63.4W   75 KT  85 MPH
96H  24/1800Z 24.5N  61.7W   70 KT  80 MPH
120H  25/1800Z 28.4N  57.7W   65 KT  75 MPH

$$
Forecaster Brown