Tropical Storm Tammy Forecast Discussion Number 7

By | October 20, 2023

WTNT45 KNHC 200246

Tropical Storm Tammy Discussion Number   7
NWS National Hurricane Center Miami FL       AL202023
1100 PM AST Thu Oct 19 2023

Tammy appears to be in the process of reorganizing. The low-level 
circulation we had been following appears to have slowed down 
tonight, as confirmed by the last few fixes from the NOAA-P3 
reconnaissance aircraft that has been in the storm tonight. In 
addition, the Tail-Doppler Radar (TDR) on board the aircraft has 
shown that the tilt between the low-level (1 km) and mid-level (5 
km) center has been reduced as a result of this slowdown. There is 
also evidence of convection beginning to wrap into the up-shear 
quadrant of the system on both GPM microwave imagery available at 
2350 UTC and recent radar images from Barbados. However, this 
restructuring has not yet resulted in an increase in the maximum 
sustained winds, which remain about 50 kt for this advisory, in 
agreement with a blend of the subjective Dvorak data and the 
earlier TDR data in the northeast quadrant. 

Recon fixes indicate the Tammy is still moving generally 
west-northwestward but slower at 290/9 kt. There is not much new to 
report from the track reasoning this cycle. An enhanced mid-level 
ridge (anomalously strong for this time of year) is beginning to 
shift eastward as a sharp mid-latitude trough approaches from the 
west. This shift should allow Tammy to turn gradually to the 
northwest and then north-northwest over the next 2-3 days. However, 
the forward motion related to this turn is forecast to be rather 
slow, related to an upper-level cutoff low south of the mid-level 
ridge weakening its steering influence on Tammy. Compared to 24 
hours ago, the global model guidance is in better agreement on this 
track solution, but continues to trend slower and a bit east of the 
prior cycle. Thus, the NHC track forecast has been adjusted quite a 
bit slower and a little further east of the prior one, blending the 
prior track with the consensus aids HCCA and TVCN. It should be 
noted that both the GFS and ECMWF remain on the east side of the 
guidance envelope, and further adjustments in that direction may be 
needed in subsequent cycles. Tammy is still forecast to recurve to 
the north-northeast by the end of the forecast period. However, 
there is a substantial amount of spread in the along-track direction 
in both the GFS and ECMWF ensembles, related to whether or not Tammy 
is fully picked up by the mid-latitude trough.

Assuming Tammy is in the process of becoming better vertically 
aligned, the storm has an opportunity to intensify as shear remains 
light to moderate (10-20 kt) and sea-surface temperatures remain 
quite warm (near 30 C). The guidance this cycle shows a bit more 
short-term intensification than before, and the NHC intensity 
forecast follows suit, showing gradual intensification to hurricane 
intensity in 36 h and a bit more intensification through day 4. 
Increasing southwesterly vertical wind shear at the end of the of 
forecast period may begin to induce weakening as Tammy moves into 
the stronger mid-latitude flow ahead of the mid-latitude trough. The 
NHC intensity forecast is close to or just a shade under the latest 
consensus aids HCCA and IVCN.


1.  Tropical storm conditions are expected to begin in portions
of the Lesser Antilles beginning Friday, where a tropical storm
warning is in effect.  Tropical storm and hurricane conditions are
possible elsewhere in the Leeward Islands where hurricane and
tropical storm watches are currently in effect. Additional watches
and warnings could be required on Friday.

2.  Heavy rains from Tammy will begin to affect the Leeward and 
northern Windward Islands on Friday, 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. 


INIT  20/0300Z 13.8N  57.1W   50 KT  60 MPH
 12H  20/1200Z 14.3N  58.2W   55 KT  65 MPH
 24H  21/0000Z 15.3N  59.9W   60 KT  70 MPH
 36H  21/1200Z 16.6N  61.3W   65 KT  75 MPH
 48H  22/0000Z 18.0N  62.0W   70 KT  80 MPH
 60H  22/1200Z 19.7N  62.5W   75 KT  85 MPH
 72H  23/0000Z 21.5N  63.0W   80 KT  90 MPH
 96H  24/0000Z 24.0N  62.0W   80 KT  90 MPH
120H  25/0000Z 28.0N  57.5W   75 KT  85 MPH

Forecaster Papin

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