Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sat Nov 24 2018 Valid 12Z Tue Nov 27 2018 - 12Z Sat Dec 01 2018 ...Pattern Overview... The medium range period begins Tuesday with a large scale upper level closed trough centered over the Great Lakes region with multiple embedded shortwaves pivoting around it, and broad troughing over the eastern Pacific with a downstream ridge axis over the Intermountain West. The large closed low slowly exits the Northeast U.S. on Thursday with quasi-zonal flow likely becoming established over much of the central U.S. and a large scale trough is expected to move inland across the western U.S. As the mid-upper level trough exits the Rockies in an emerging/energetic southern stream, the flow will become increasingly favorable for Plains/MS Valley system genesis to open December. This would be coincident with the building of downstream ridge over the eastern U.S. ...Model Guidance and Uncertainty Assessment... The WPC medium range product suite was primarily derived from a composite blend of the now quite reasonably well clustered guidance over the lower 48 from the 00 UTC GEFS/NAEFS/ECMWF ensemble means and latest GFS/ECMWF determinitic models through Thu. Increasing forecast spread and run to run inconsistencies, especially over the eastern Pacific then suggested usage of only these still reasonably compatable ensemble means into next Fri/Sat (Days 6/7). ..Weather Highlights/Threats... Inland wrapping heavy snow should exit northern New England Wednesday, but low level cold air advection/high winds in the wake of the system will support lingering lake effect snows into Thursday. Out West, the heaviest precipitation should focus from the Pacific Northwest to central CA as two main energetic Pacific storm systems move onshore and bring enhanced moisture into these areas, with an atmospheric river event possible by Tuesday into northern California. In addition to the heavy rainfall and local runoff threat, heavy snow is likely for the higher mountains from the Cascades and Sierra to the central and northern Rockies. Subsequent system progression/genesis over the central U.S. into next weekend may lead to pronounced lead Gulf moisture inflow to fuel an expanding area of precipitation. Temperatures are expected to be on the order of 15 to 20 degrees below average from the northern plains to the Ohio Valley on Tuesday and Wednesday before a moderation trend ensues by later in the week. Areas with fresh snow cover across the Midwest in the wake of the winter storm may have overnight lows dropping well into the single digits for the early to middle part of the upcoming work week, with some readings possibly falling below zero where winds are light and skies clear out. Temperatures should generally be within 5-10 degrees of climatological averages for the western third of the nation. Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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