[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 210841
TCDAT5

Hurricane Tammy Discussion Number  12
NWS National Hurricane Center Miami FL       AL202023
500 AM AST Sat Oct 21 2023

Mosaic radar data from Martinique, Guadeloupe, and Barbados show
that Tammy's core is passing just to the east of Martinique and
Dominica.  The eyewall has become more pronounced during the past
several hours although it is open on the south side.  In infrared
satellite imagery, the hurricane has a relatively small CDO and a
prominent trailing convective band to the south, but no evidence of
an eye.  Based on earlier Air Force Reserve reconnaissance data,
Tammy's initial intensity remains 70 kt.

Aircraft fixes and the radar data indicate that Tammy has finally
turned northwestward at 305/8 kt.  A strong mid-latitude trough
moving off the east coast of the United States is eroding the
western portion of the Atlantic subtropical ridge, which should
cause Tammy to continue northwestward across the Leeward Islands
through tonight and early Sunday, and then turn north-northwestward
and northward Sunday and Monday.  For the first 3 days of the
forecast, the new NHC track prediction is nearly on top of the
previous forecast and is close to the HCCA consensus aids.  There
has been a dramatic change in the global model solutions beyond day
3.  Some models (the ECMWF and UKMET) show the trough completely
bypassing Tammy and leaving the cyclone to stall north of Puerto
Rico and the Virgin Islands, while others (the GFS and Canadian)
initially show Tammy turning northeastward but then interacting
with a shortwave trough and slowing down over the western Atlantic.
The NHC track forecast has been shifted significantly westward on
days 4 and 5 to account for these latest model trends.

Tammy will continue moving over very warm waters of 29-30 degrees
Celsius during the next few days, but the storm will also continue
to contend with some moderate westerly or southwesterly vertical
shear.  Fluctuations in intensity will be possible during the next
few days depending on if the oceanic or atmospheric factors win
out, but all in all little change in strength is predicted during
the next 3 to 4 days.  Some weakening is possible at the end of the
forecast period due to a more significant increase in shear.


KEY MESSAGES:

1.  Hurricane and tropical storm conditions are expected in portions
of the Leeward and northern Windward Islands through Sunday.

2.  Heavy rains from Tammy will affect the Windward and Leeward
Islands today, 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  21/0900Z 15.2N  60.4W   70 KT  80 MPH
 12H  21/1800Z 16.3N  61.3W   75 KT  85 MPH
 24H  22/0600Z 17.8N  62.4W   75 KT  85 MPH
 36H  22/1800Z 19.3N  63.3W   75 KT  85 MPH
 48H  23/0600Z 20.6N  64.0W   75 KT  85 MPH
 60H  23/1800Z 21.9N  64.2W   75 KT  85 MPH
 72H  24/0600Z 22.8N  64.0W   75 KT  85 MPH
 96H  25/0600Z 24.5N  63.1W   70 KT  80 MPH
120H  26/0600Z 27.0N  62.1W   60 KT  70 MPH

$$
Forecaster Berg
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