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| #1019966 (Received by flhurricane at: 5:06 AM 05.Oct.2020) TCDAT5 Tropical Storm Gamma Discussion Number 12 NWS National Hurricane Center Miami FL AL252020 400 AM CDT Mon Oct 05 2020 Gamma is a strongly sheared tropical storm that has been meandering just north of the northern coast of the Yucatan Peninsula of Mexico. Strong convective cloud tops colder than -80 deg C are located more than 60 nmi north and northeast of the well-defined and fully exposed low-level circulation center. An Air Force Reserve reconnaissance aircraft investigating the cyclone found maximum 850-mb flight-level winds of only 44 kt along with peak SFMR surface winds of 47 kt. In addition, a 0329Z ASCAT-A pass revealed peak surface winds of 41 kt. Allowing for undersampling by the scatterometer instrument and blending it with the SFMR winds supports lowering the initial intensity to 45 kt. The central pressure of 999 mb is based on a center dropsonde reports of 1000 mb and 10-12 kt winds. The initial motion estimate is an uncertain 240/02 kt. Gamma`s future track remains highly uncertain after day 2. For the next 48 hours or so, the cyclone is forecast to move slowly southwestward to west-southwestward and weaken due to strong southerly vertical wind shear. Thereafter, TD-26 to the east is forecast to strengthen into a hurricane, accompanied by a corresponding increase in the size of that cyclone`s circulation, which is expected to result in some binary interaction between the two tropical cyclones. Due to Gamma`s expected smaller size on days 3 and 4, the cyclone is forecast to be pulled eastward and then northward, making landfall on and then moving over the northwestern Yucatan Peninsula. By day 5, Gamma is expected to degenerate into a remnant low and then be absorbed by the larger circulation of the expected central Gulf of Mexico hurricane that is currently TD-26. The new NHC track forecast is similar to but a little slower than the previous advisory track, and lies close to the southern edge of the guidance envelope. Strong southerly wind shear, combined with drier and more stable air just to the west of Gamma, is expected to weaken the cyclone further over the next few days. If land interaction occurs sooner than expected, then the cyclone could weaken and dissipate sooner than currently forecast. For now, the intensity forecast maintains continuity with the previous advisory and shows dissipation by 120 hours as Gamma interacts with what is expected to be a significant hurricane over the central Gulf of Mexico by 96 hours, which could end up absorbing the smaller Gamma tropical cyclone. Due to the uncertainty in the forecast at this time, future advisories may feature larger-than-normal changes to the track or intensity forecast. It is also worth noting that several dynamical model trackers appear to lose Gamma and jump to tracking Tropical Depression Twenty-Six when it reaches the Gulf of Mexico, resulting in unrealistic depictions of the model intensity and track forecasts. 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 05/0900Z 22.7N 87.5W 45 KT 50 MPH 12H 05/1800Z 22.5N 88.0W 40 KT 45 MPH 24H 06/0600Z 22.1N 88.9W 40 KT 45 MPH 36H 06/1800Z 21.4N 89.8W 35 KT 40 MPH 48H 07/0600Z 20.8N 90.4W 35 KT 40 MPH 60H 07/1800Z 20.3N 90.5W 30 KT 35 MPH...INLAND YUCATAN 72H 08/0600Z 20.7N 90.1W 30 KT 35 MPH...INLAND YUCATAN 96H 09/0600Z 22.5N 90.0W 30 KT 35 MPH...OVER WATER 120H 10/0600Z...DISSIPATED $$ Forecaster Stewart |