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Hurricane Tammy Discussion Number  21
NWS National Hurricane Center Miami FL       AL202023
1100 AM AST Mon Oct 23 2023
 
Tammy appears to be holding steady in spite of the moderate 
southwesterly vertical wind shear.  There have been regular bursts 
of deep convection over the center and a fragmented band around the 
eastern portion of the circulation.  The latest intensity is held at 
70 kt to represent a blend of the subjective satellite intensity 
estimates from TAFB and SAB, which were 77 kt and 65 kt, 
respectively.

The hurricane is moving northward at 005/6 kt.  Tammy should 
continue to turn north-northeastward to northeastward in the flow 
between a mid-latitude trough over the western Atlantic and a 
mid-level ridge over the central subtropical Atlantic.  Beyond 60 h, 
there is still a significant spread in the model guidance.  Most of 
the regional models and the GFS model forecast Tammy to continue 
northward around the western side of the ridge.  A different cluster 
of model solutions, including the ECMWF, merge Tammy with a cutoff 
low that forms from the trough and turns the cyclone northwestward 
to westward.  The latest NHC track forecast favors the latter 
scenario and has been nudged slightly north of the previous 
prediction.

Southwesterly shear appears to be influencing Tammy's structure.  
The vertical wind shear is expected to persist and likely increase 
in about 36 h, forcing some dry mid-level air into the cyclone's 
circulation.  These environmental conditions should lead to gradual 
weakening.  Model guidance predicts that Tammy should undergo 
structural changes over the next couple of days as it interacts with 
the aforementioned cutoff low.  The latest official intensity 
forecast now shows Tammy becoming extratropical in 72 hours. 
 
 
KEY MESSAGES:
 
1.  Heavy rain from Tammy will remain possible over the British 
Virgin Islands and Leeward Islands through today.  This rainfall may 
produce isolated flash and urban flooding, along with isolated 
mudslides in areas of higher terrain.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/1500Z 21.9N  63.9W   70 KT  80 MPH
 12H  24/0000Z 22.7N  63.4W   70 KT  80 MPH
 24H  24/1200Z 23.6N  62.4W   75 KT  85 MPH
 36H  25/0000Z 24.5N  61.2W   80 KT  90 MPH
 48H  25/1200Z 25.8N  59.9W   75 KT  85 MPH
 60H  26/0000Z 27.3N  58.9W   70 KT  80 MPH
 72H  26/1200Z 29.1N  58.6W   60 KT  70 MPH...POST-TROP/EXTRATROP
 96H  27/1200Z 31.2N  60.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  28/1200Z 32.4N  63.6W   45 KT  50 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Bucci
 
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