[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 210258
TCDAT5

Hurricane Tammy Discussion Number  11
NWS National Hurricane Center Miami FL       AL202023
1100 PM AST Fri Oct 20 2023

Although the satellite and radar presentations of Tammy do not
appear to have become significantly better organized, with a small
CDO and limited convective banding features, reconnaissance
aircraft data indicate the hurricane has strengthened slightly.
Dropsonde and SFMR-observed surface winds from Air Force and NOAA
Hurricane Hunter aircraft support an intensity of 70 kt for this
advisory.  This is a little above the most recent Dvorak estimates.

Center fixes from the Hurricane Hunter aircraft indicate that Tammy
is still moving west-northwestward or at about 295/8 kt, so the
anticipated northwestward turn has not yet occurred.  The hurricane
is moving along the southwestern periphery of a large subtropical
high pressure area.  A strong mid-tropospheric trough moving off the
U.S. east coast should erode the western portion of the ridge and
cause Tammy to turn northwestward to northward during the next
couple of days.  However, based on the more westward motion observed
by the Hurricane Hunters, the official track forecast has been
shifted somewhat to the west of the previous one through 48 hours.
This is a little west of the model consensus.  The track guidance is
not in very good agreement on the movement of Tammy after is passes
the northern Leeward Islands.  Some of the models, such as the
ECMWF, show the system moving quite slowly at 72 hours and beyond.
The GFS is an outlier with a more rapid north-northeastward motion.
Based on the dynamical model consensus, the NHC track forecast is
slower than the previous one in the 3- to 5-day time frame.

Tammy should remain in an environment of high oceanic heat content,
but with moderate vertical wind shear for the next couple of days.
Thus, some modest strengthening is forecast during the next 24
hours, while the system passes near or over portions of the Leeward
Islands.  The NHC intensity forecast is a little higher than the
previous one, and above the simple and corrected model consensus
guidance.

It should be noted that there is a significant area of deep
convection well to the south-southeast of the center which
could bring heavy rains to portions of the Lesser Antilles well
after the center of the hurricane passes by.


KEY MESSAGES:

1.  Hurricane conditions are expected in portions of the Leeward
Islands by late tonight through Saturday. Tropical storm
conditions are expected to begin in portions of the Lesser Antilles
within the warning area overnight.

2.  Heavy rains from Tammy will affect the Windward and Leeward
Islands into Saturday morning, spreading into the British and U.S.
Virgin Islands and eastern Puerto Rico as early as Sunday.  This
rainfall may produce isolated flash and urban flooding, along with
isolated mudslides in areas of higher terrain.

3. A storm surge could produce coastal flooding in areas of onshore
winds as the center of Tammy moves near or over the Leeward Islands.
Near the coast, the surge will be accompanied by large and dangerous
waves.


FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 14.7N  60.2W   70 KT  80 MPH
 12H  21/1200Z 15.8N  61.0W   75 KT  85 MPH
 24H  22/0000Z 17.3N  62.0W   80 KT  90 MPH
 36H  22/1200Z 18.8N  63.0W   80 KT  90 MPH
 48H  23/0000Z 20.0N  63.8W   80 KT  90 MPH
 60H  23/1200Z 21.4N  64.3W   80 KT  90 MPH
 72H  24/0000Z 22.4N  64.0W   75 KT  85 MPH
 96H  25/0000Z 24.0N  62.5W   70 KT  80 MPH
120H  26/0000Z 27.0N  60.0W   65 KT  75 MPH

$$
Forecaster Pasch
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