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WTNT45 KNHC 200856

TCDAT5

 

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