Tropical Storm Tammy Discussion Number   8
NWS National Hurricane Center Miami FL       AL202023
500 AM AST Fri Oct 20 2023

Tammy is still an asymmetric tropical storm, although the surface
center is embedded beneath a persistent area of deep convection.  
Radar data from Barbados also suggest that the circulation remains 
tilted from west to east with height.  An Air Force Reserve 
Hurricane Hunter aircraft investigated Tammy a few hours ago and 
measured maximum 850-mb flight-level winds of 61 kt and SFMR 
surface winds around 50 kt, indicating that the storm's intensity 
remains 50 kt.  The central pressure has fallen slightly to 1000 mb.

The aircraft center fixes indicate that Tammy has continued to slow 
down and is moving toward the west-northwest (290 degrees) at 7 kt. 
An amplified deep-layer trough moving across the eastern United 
States is beginning to push the Atlantic subtropical ridge 
eastward, and this pattern evolution should allow Tammy to turn 
northwestward by this evening, with its center passing very near or 
over the Leeward Islands tonight and on Saturday.  The new NHC 
forecast is very close to the previous track prediction during the 
first 36 hours and lies near the HCCA and TVCA consensus aids.  
After 36 hours, the track guidance has shifted a bit west and 
slowed down, apparently due to the aforementioned trough lifting to 
the northeast and leaving Tammy behind.  As a result, the official 
forecast continues to show recurvature over the central Atlantic by 
day 5, but at a relatively slow forward speed.

SHIPS guidance suggests that deep-layer shear over Tammy should 
decrease a bit over the next 24 hours, but model fields indicate 
there could be some mid-level shear below the outflow level.  
Still, environmental conditions appear conducive to support gradual 
strengthening, and the NHC intensity forecast brings Tammy to 
hurricane intensity by 36 hours as the center passes near or over 
the Leeward Islands.  It is possible that Tammy could become a 
hurricane before that time, as suggested by several models 
including SHIPS, HAFS-B, HWRF, and the HCCA corrected consensus.  A 
peak in intensity is forecast in 3-4 days, before southwesterly 
vertical shear increases in earnest ahead of the trough.  Model 
fields suggest that extratropical transition could begin toward the 
end of the forecast period, but for now Tammy is still shown as a 
hurricane on day 5.


KEY MESSAGES:

1.  Tammy is expected to strengthen to a hurricane by Saturday 
while it moves near or over portions of the Leeward Islands.  
Tropical storm conditions are expected to begin in portions of the 
Lesser Antilles within the tropical storm warning area this 
afternoon or evening, with hurricane conditions possible within the 
hurricane watch area on Saturday.  Additional watches and warnings 
could be required later today.

2.  Heavy rains from Tammy will begin to affect the Leeward and 
northern Windward Islands today, 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  20/0900Z 14.0N  58.3W   50 KT  60 MPH
12H  20/1800Z 14.7N  59.4W   55 KT  65 MPH
24H  21/0600Z 15.9N  60.7W   60 KT  70 MPH
36H  21/1800Z 17.3N  61.8W   65 KT  75 MPH
48H  22/0600Z 18.8N  62.6W   70 KT  80 MPH
60H  22/1800Z 20.3N  63.2W   75 KT  85 MPH
72H  23/0600Z 21.7N  63.4W   80 KT  90 MPH
96H  24/0600Z 24.3N  61.9W   80 KT  90 MPH
120H  25/0600Z 27.8N  58.8W   75 KT  85 MPH

$$
Forecaster Berg