####018004599####
WTNT45 KNHC 200246

TCDAT5

 

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.

 

KEY MESSAGES:

 

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. 

 

 

FORECAST POSITIONS AND MAX WINDS

 

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