Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Wed Nov 27 2019 Valid 12Z Sat Nov 30 2019 - 12Z Wed Dec 04 2019 ...Major Winter Storm to impact the Northern Plains to the Northeast during the Thanksgiving weekend... ...Pattern overview and Guidance/Predictability Assessment... The medium range period (Saturday-Wednesday) begins with shortwave energy ejecting into the Central U.S. inducing cyclogenesis in the lee of the Rockies. This results in a deep low pressure system and major winter storm tracking from the north-central Plains to the Great Lakes this weekend. Additional southern stream energy rounding the base of this cold core upper low allows for a new surface low to form off the Mid-Atlantic coast early Monday which then tracks up the New England Coast through Tuesday. Models and ensembles continue to show fairly good agreement on the track and intensity for this storm, with only minor timing and intensity differences remaining. A general model compromise worked well and shows good continuity with the previous shift. Meanwhile, energies dropping southward from the Gulf of Alaska establishes another trough into or just off the West Coast by day 4/Sunday. For days 3-4, there is good enough model agreement to warrant a mostly deterministic blend between the latest runs of the GFS and ECMWF. After this time, there remains question on the evolution of an upper level low off the West Coast. The latest run of the GFS (and CMC) show an upper low lingering through most of the day 5-7 period well off the Southwest Coast, but run-to-run continuity in the GFS is poor with this. The ECMWF shows better run-to-run continuity in showing an upper low shifting southward along the coast and eventually into the Southwest on day 7. The ensemble means also seem to support this idea of weakening the low and bringing it inland as well. Therefore, for days 5-7, increasing weighting of the ensemble means was used with a good chunk of the ECMWF which is the deterministic model closest to the ensemble means. ...Weather Highlights/Threats... A major winter storm with widespread hazardous weather is expected for parts of the Central and Eastern U.S. over the Thanksgiving holiday weekend. To the north and west of the surface low, heavy and significant accumulating snow is likely from the Dakotas to the Upper Great Lakes on Saturday, with accumulating snow likely shifting into much of the Northeast Sunday into Monday. High winds will also accompany this storm, making for difficult and hazardous travel across much of the region. To the south, locally heavy rainfall is possible along the attendant cold front as it shifts eastward across the Deep South/Ohio Valley/Mid-Atlantic states this weekend. Severe thunderstorms are also possible across the Lower Mississippi Valley on Saturday as indicated by the latest medium range SPC severe outlook. In the West, expect another round of heavy mountain snows across the higher terrain of northern and Central California, with rain in the lower elevations as well as up the Pacific Northwest Coast. More rain should also shift into parts of Southern California by Tuesday as well. Temperatures remain cold across the West Saturday into Sunday with daytime highs 10 to 20 degrees below normal in some spots. Temperatures should moderate back towards normal (though still slightly below) through the rest of the period. Ahead of the cold front in the Central U.S., warmer than average temperatures are expected Saturday, but should quickly get replaced with below normal temperatures by Monday which also sweep into the East Tuesday/Wednesday following the cold frontal passage. 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