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Tropical Storm Gamma Forecast Discussion Number 11
2020-10-05 04:39:22| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sun Oct 04 2020 000 WTNT45 KNHC 050239 TCDAT5 Tropical Storm Gamma Discussion Number 11 NWS National Hurricane Center Miami FL AL252020 1000 PM CDT Sun Oct 04 2020 Gamma has moved much farther northeast than previously anticipated. With little in the way of low or mid-level large-scale steering flow currently in place, it seems likely that upper-level southwesterlies, also responsible for the shear affecting the tropical storm, are causing this recent northeastward motion. This may be either through reformations of the center closer to the convection or a direct contribution to the net steering. An Air Force Reserve Hurricane Hunter aircraft is currently investigating Gamma and so far has found max SFMR winds of 47 kt but flight level winds of only 45 kt. Given the data so far, there is no indication that Gamma is any stronger than the 50 kt intensity, but it seems prudent to let the plane finish its pattern before lowering the winds at this time. Gamma's future track is highly uncertain. The track guidance spread is much higher than usual, and confidence in the forecast is accordingly low. Most of the global models indicate that Gamma will soon begin moving generally west-southwestward as a mid-level ridge briefly builds over the eastern and central Gulf of Mexico. Unfortunately, there is little agreement on exactly when this will happen or how fast Gamma will move once it turns. After about 48 h, Gamma could interact with Tropical Depression Twenty-Six, which is forecast to move over the northwestern Caribbean Sea at that time. Current dynamical models are notoriously bad at forecasting such interactions, but if the two systems do interact it will likely cause Gamma to move inland over the Yucatan. Whether such an interaction occurs or not, continued shear and proximity to land should cause the tropical storm to gradually weaken, and Gamma could dissipate if it moves over land (and stays there) in a few days, as shown by the GFS. The ECMWF shows the vortex remaining over water but dissipating nonetheless, while a couple other global models maintain the vortex longer and show it moving north over the central Gulf of Mexico late in the forecast period. The NHC forecast is a compromise between those solutions, showing a remnant low moving north over the southern Gulf of Mexico by day 5. In general, the NHC intensity forecast is similar to the previous one, but slightly lower to account for the system potentially moving inland. 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 intensity and track forecasts. 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 just offshore, tropical storm conditions are likely along portions of the northern coast of the Yucatan Peninsula on Monday. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 22.8N 87.2W 50 KT 60 MPH 12H 05/1200Z 22.7N 87.7W 45 KT 50 MPH 24H 06/0000Z 22.3N 88.5W 45 KT 50 MPH 36H 06/1200Z 21.8N 89.5W 40 KT 45 MPH 48H 07/0000Z 21.3N 90.3W 40 KT 45 MPH 60H 07/1200Z 20.3N 90.7W 35 KT 40 MPH 72H 08/0000Z 20.2N 90.1W 30 KT 35 MPH...INLAND 96H 09/0000Z 22.0N 90.7W 30 KT 35 MPH...OVER WATER 120H 10/0000Z 24.0N 91.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky
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