[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 040855
TCDAT5

Tropical Storm Gamma Discussion Number   8
NWS National Hurricane Center Miami FL       AL252020
400 AM CDT Sun Oct 04 2020

Tropical Storm Gamma has moved back over open water, just offshore
the northern Yucatan Peninsula in the extreme southern Gulf of
Mexico. Although deep convection has been increasing near the
center, the overall convective cloud shield is now strung out to
the north and northeast due to southerly to southwesterly vertical
wind shear in excess of 15 kt. The initial intensity of 45 kt is
based on an earlier ASCAT pass showing 39-kt surface winds just
offshore the northeastern tip of Yucatan, and these winds were
likely under-sampled due to their proximity to the coast. Satellite
classifications from both TAFB and SAB were T3.0/45 kt, which
further supports the 45-kt intensity assessment.

The initial motion estimate is 355/04 kt. As the day progresses,
Gamma is expected to move further into a slight weakness in
subtropical ridge located to the north of the cyclone and slow down,
possibly stalling by this afternoon. The trough over the
southeastern U.S. that is creating the weakness is forecast to move
offshore the U.S. east coast by tonight, causing the ridge to build
back in and force Gamma westward to west-southwestward from Monday
through Thursday. The new NHC track forecast follows the TVCA and
FSSE consensus models, and is similar to but slightly slower and a
little to the right/north of the previous advisory track.

Now that Gamma is back out over open water, some slight
re-strengthening is expected today. However, strong southerly
vertical wind shear is forecast to increase across the cyclone by
tonight and especially on Monday, and continue for the next few
days. The result should be a slow but gradual weakening trend on
Monday through the end of the forecast period. The new official
intensity forecast follow the downward trend of the simple- and
corrected-consensus models, but is slightly higher than the IVCN,
HCCA, FSSE intensity models.

KEY MESSAGES:

1. Gamma is expected to produce heavy rainfall for several days over
portions of southeastern Mexico, the Yucatan Peninsula, Central
America, and far western Cuba.  This rainfall could result in
life-threatening flash flooding and mudslides, particularly in the
mountainous regions of southeastern Mexico and Central America.

2. Even though Gamma is offshore, tropical storm conditions will
continue across portions of the Yucatan Peninsula today.


FORECAST POSITIONS AND MAX WINDS

INIT  04/0900Z 22.0N  88.2W   45 KT  50 MPH
 12H  04/1800Z 22.6N  88.3W   50 KT  60 MPH
 24H  05/0600Z 22.7N  88.6W   45 KT  50 MPH
 36H  05/1800Z 22.4N  89.4W   45 KT  50 MPH
 48H  06/0600Z 22.0N  90.2W   40 KT  45 MPH
 60H  06/1800Z 21.5N  90.9W   40 KT  45 MPH
 72H  07/0600Z 21.0N  91.6W   40 KT  45 MPH
 96H  08/0600Z 20.4N  92.5W   40 KT  45 MPH
120H  09/0600Z 20.4N  93.4W   40 KT  45 MPH

$$
Forecaster Stewart
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