Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Nov 26 2019 Valid 12Z Fri Nov 29 2019 - 12Z Tue Dec 03 2019 ...Major winter storm likely for the Thanksgiving Holiday Period... ...Pattern overview and Guidance/Predictability Assessment... The medium range period (Friday to Tuesday) begins with a broad upper level low within very amplified troughing over the West Coast. Shortwave energy rounding the base of the trough ejects out into the Central Plains by late Friday-early Saturday with a rather deep upper level low developing in the Northern Plains and tracking quickly eastward through the weekend. Compared to recent days, the models have come into much better agreement with this feature, with some mostly minor timing and intensity differences remaining. The notable outlier continues to be the UKMET which is almost a day later to eject the energy into the Plains than the rest of the guidance. This results in a rather potent surface low, and potentially major winter storm, tracking from the north-central Plains to the Upper Great Lakes/Midwest, and off the Northeast coast by Monday. Behind this main system, upper ridging builds across the Central U.S. while reinforced troughing gets established just west of the West Coast. There is enough run-to-run variability in the deterministic models with the evolution of various embedded vorts that an ensemble mean blend is the best approach at this point. Across the board, this cycle of the WPC progs used a blend of the GFS/ECMWF with the ECENS/GEFS means. Good agreement with the major Central U.S. upper low warranted a majority deterministic weight days 3-5, with increasing usage of the means thereafter. This maintains good continuity with the previous WPC forecast. ...Weather Highlights/Threats... The Central U.S. storm will bring a variety of weather hazards to a large part of the country for the upcoming Thanksgiving holiday weekend. Heavy mountain snows will remain possible on Friday for much of the Central and Southern Rockies coming to an end by Saturday as the system shifts east. As the surface low moves into the Plains, a swath of heavy to potentially significant accumulating snowfall is likely north and west of the low from the Northern Plains to the Upper Great Lakes Friday and Saturday, shifting into New England on Sunday. High winds may also accompany this system making for difficult and hazardous travel across much of the region. Along and ahead of the attendant cold front, heavy to possibly excessive rainfall is likely from the Southern Plains/Gulf Coast to the Ohio Valley. The latest SPC severe weather outlook also shows a 15 percent chance of severe weather for Friday across the Southern Plains. The greatest temperature anomalies will focus across the West through the weekend with daytime highs 20 to 25 degrees below normal for some but should trend back towards normal (though still slightly below) by the end of the period. The Midwest to Deep South are warm Friday and Saturday, with cooler than average temperatures moving in by late weekend and early next week. Cold front exiting the East day 6/Tuesday will bring a return to below normal temperatures. Santorelli Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml