[Tropical] Tropical Cyclone Discussion

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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
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