Tropical Storm Gamma Forecast Discussion


000
WTNT45 KNHC 051455
TCDAT5

Tropical Storm Gamma Discussion Number  13
NWS National Hurricane Center Miami FL       AL252020
1000 AM CDT Mon Oct 05 2020

Air Force Reserve reconnaissance aircraft investigating Gamma found 
maximum 850 mb flight-level winds of 37 kt and surface SFMR peak 
winds of 40 kt, and the central pressure has increased to 1003 mb.  
Therefore, the initial intensity is lowered to 40 kt for this 
advisory. 

Stiff, persistent southerly shear, and an inhibiting thermodynamic
surrounding environment should prevent Gamma from restrengthening, 
although, the Decay SHIPS intensity model does indicate a very 
brief period of decreasing shear magnitude just prior to landfall 
around the 24 hour period.  Afterward, the shear, once again, 
increases.  Gamma is expected to make landfall over the 
northwestern Yucatan peninsula on Tuesday and further weakening 
to a remnant low is forecast Wednesday as the cyclone remains 
inland over the northern Mexican State of Yucatan.  Dissipation is 
likely to occur Friday over the region, if not sooner.

The initial motion is estimated to be a south-southwestward drift, 
or 205/2 kt. A turn southwestward or west-southwestward is 
expected by tonight and this general motion should continue through 
Wednesday morning while moving inland over the northern Mexican 
State of Yucatan.  The large-scale models still insist on some 
binary interaction occurring with Tropical Storm Delta commencing 
Tuesday night/Wednesday morning.  This should cause Gamma to 
gradually turn cyclonically through Thursday while remaining over 
the Northern Yucatan peninsula.  If there is a delay in Gamma's 
landfall, or the cyclone does remain just offshore, similar to the 
outlier UKMET, then Tropical Storm Delta could end up absorbing the 
smaller Gamma tropical cyclone toward the end of the week.  For 
now, the NHC forecast will side with a landfall scenario which 
agrees with the HCCA consensus and the GFS/ECMWF solutions.

KEY MESSAGES:

1. Gamma is expected to produce heavy rainfall through midweek over 
portions of southeast Mexico, including the Mexican states of 
Yucatan, Campeche, and Tabasco. This rainfall could result in 
significant flash flooding. 

2. Even though Gamma is just offshore, tropical storm conditions
are likely along portions of the northern coast of the Yucatan
Peninsula later today.


FORECAST POSITIONS AND MAX WINDS

INIT  05/1500Z 22.3N  87.6W   40 KT  45 MPH
 12H  06/0000Z 22.1N  88.4W   40 KT  45 MPH
 24H  06/1200Z 21.6N  89.2W   35 KT  40 MPH
 36H  07/0000Z 20.9N  89.9W   30 KT  35 MPH...INLAND
 48H  07/1200Z 20.2N  90.2W   25 KT  30 MPH...POST-TROP/INLAND
 60H  08/0000Z 20.3N  89.8W   25 KT  30 MPH...POST-TROP/INLAND
 72H  08/1200Z 21.0N  89.5W   20 KT  25 MPH...POST-TROP/INLAND
 96H  09/1200Z...DISSIPATED

$$
Forecaster Roberts

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