652
WTNT45 KNHC 030905
TCDAT5

Tropical Storm Gamma Discussion Number 4
NWS National Hurricane Center Miami FL AL252020
400 AM CDT Sat Oct 03 2020

Satellite imagery and reports from an Air Force Reserve Hurricane
Hunter aircraft indicate that Gamma is strengthening. A central
dense overcast with cloud tops to -85C has formed, along with an
outer convective band in the eastern semicircle. The aircraft has
reported maximum 850 mb flight-level winds of 58 kt and reliable-
looking surface wind estimates from the Stepped Frequency Microwave
Radiometer of 40-45 kt. Based on this, the initial intensity is
increased to 45 kt.

The initial motion is again a little to the left of the previous
advisory, 305/8 kt. During the next 24-36 h, Gamma is expected to
turn northward as it tries to recurve into a break in the
subtropical ridge caused by a mid-latitude trough over the eastern
United States. The track guidance, however, is in good agreement
that the trough will bypass the tropical cyclone, with a ridge
building to the north after 36 h. This should cause Gamma to turn
westward or southwestward between the ridge and the Central
American cyclonic gyre to the south. There is uncertainty as to
whether Gamma will move over the Gulf of Mexico before this turn
occurs, as the ECMWF keeps the center inland or very near the
coast. The new forecast track is north of the ECMWF, but a little
to the south of the previous track. The foreast track calls for
landfall on the northeastern Yucatan Peninsula in about 12 h,
followed by the center reaching the Gulf of Mexico in about 36 h
and a track near the coast of the Yucatan Peninsula thereafter.

Conditions appear favorable for strengthening until landfall, and
the forecast 12-h intensity of 50 kt could be conservative given
current trends. The remainder of the intensity forecast is quite
uncertain due to the possibility of land interaction, dry air
entrainment, shear, and possible interaction with another system to
the east. The intensity guidance again does not show much
strengthening if Gamma emerges into the Gulf of Mexico, and that
part of the new intensity forecast has only minor adjustments from
the previous one. It should be noted that several models show a low
pressure area over the northern Gulf of Mexico after 120 h. Whether
this will be Gamma or a new system is unclear at this time.

The biggest threat from this large and slow-moving storm is the
the potential for heavy rainfall and flooding, particularly near
and over mountainous terrain.


KEY MESSAGES:

1. Gamma is expected to produce heavy rainfall that could result in life-threatening flash flooding over portions of the Yucatan
Peninsula, far western Cuba and well away from the center in
the Mexican states of Campeche, Tabasco, and northern Chiapas.

2. Gamma is forecast to bring tropical storm conditions to portions
of the Yucatan Peninsula today and Sunday, where a Tropical Storm
Warning is in effect.


FORECAST POSITIONS AND MAX WINDS

INIT 03/0900Z 19.4N 86.9W 45 KT 50 MPH
12H 03/1800Z 20.2N 87.4W 50 KT 60 MPH
24H 04/0600Z 21.1N 87.9W 45 KT 50 MPH...INLAND
36H 04/1800Z 21.7N 88.0W 40 KT 45 MPH...OVER WATER
48H 05/0600Z 21.8N 88.4W 40 KT 45 MPH
60H 05/1800Z 21.6N 89.3W 45 KT 50 MPH
72H 06/0600Z 21.2N 90.5W 45 KT 50 MPH
96H 07/0600Z 20.5N 92.4W 45 KT 50 MPH
120H 08/0600Z 19.5N 93.5W 40 KT 45 MPH

$$
Forecaster Beven
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