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| #1019834 (Received by flhurricane at: 5:02 AM 04.Oct.2020) 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 |