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| #1019723 (Received by flhurricane at: 5:08 AM 03.Oct.2020) 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 |