[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 182044
TCDAT5

Tropical Storm Tammy Discussion Number   1
NWS National Hurricane Center Miami FL       AL202023
500 PM AST Wed Oct 18 2023

The tropical disturbance that NHC has been monitoring for many days
as it traversed the tropical Atlantic (AL94) has finally become
sufficiently organized, both convectively and circulation-wise, to
be designated as a tropical cyclone.  The circulation still appears
a bit elongated in visible satellite images, but the convective
activity suggests that a well-defined center has formed.  Earlier
scatterometer data showed winds of 30-35 kt to the northeast of the
center, and TAFB provided a Dvorak estimate of T2.5/35 kt.
Therefore, advisories are being initiated on Tropical Storm Tammy,
with an initial intensity of 35 kt.

The initial motion is westward (275 degrees) at a fast 20 kt, but
there is uncertainty in this estimate given that the center has
likely just recently formed.  A strong mid-level ridge to the north
is forecast to keep Tammy on a westward motion, but slower, over the
next 24 hours or so.  After that, a deep-layer trough moving across
the eastern United States is expected to push the ridge eastward,
allowing Tammy to turn toward the northwest and then north into the
weekend.  The track models agree on this general scenario, with
Tammy moving over or near the Leeward Islands Friday and Saturday.
However, there are some notable differences, with some of the
stronger models (e.g., HWRF and GFS) showing a turn just before
Tammy reaches the islands, while the weaker models (e.g., HAFS and
HMON) move the storm farther west into the northeastern Caribbean
Sea.  The initial NHC track forecast is between these two periphery
scenarios, and is close to the ECMWF, TVCA, and HCCA solutions.
After passing the Leeward Islands, Tammy is expected to
accelerate northeastward over the central Atlantic ahead of the
trough.

Global model fields and SHIPS model diagnoses suggest that Tammy may
deal with some vertical shear, and possibly some dry air in the
vicinity, over the next few days.  On the other hand, the storm will
be moving over very warm waters of 29-30 degrees Celsius.
Therefore, gradual strengthening is forecast during the next couple
of days, with the NHC intensity prediction very close to the IVCN
and HCCA aids.  While the forecast depicts a 55-kt tropical storm
moving across the islands, there could be adjustments to this
forecast once Tammy's current intensity and structure become
clearer, and users should be prepared for possible forecast changes.
After Tammy passes the Leeward Islands, the intensity models suggest
that some further intensification will be possible as it accelerates
northeastward over the central Atlantic.


KEY MESSAGES:

1.  Tropical storm conditions are possible across portions of the
Lesser Antilles beginning on Friday.  Tropical storm watches are
currently in effect for Barbados, Dominica, Martinique, and
Guadeloupe, and additional watches or warnings will likely be
required tonight or on Thursday.

2.  Heavy rains from Tammy will begin to affect the northern
Windward and Leeward 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  18/2100Z 13.0N  51.7W   35 KT  40 MPH
 12H  19/0600Z 13.1N  53.7W   40 KT  45 MPH
 24H  19/1800Z 13.5N  55.9W   50 KT  60 MPH
 36H  20/0600Z 14.1N  58.2W   55 KT  65 MPH
 48H  20/1800Z 15.0N  60.2W   55 KT  65 MPH
 60H  21/0600Z 15.9N  61.7W   55 KT  65 MPH
 72H  21/1800Z 17.5N  62.7W   55 KT  65 MPH
 96H  22/1800Z 21.4N  63.4W   60 KT  70 MPH
120H  23/1800Z 26.1N  60.2W   60 KT  70 MPH

$$
Forecaster Berg
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