Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Dec 27 2020 Valid 12Z Wed Dec 30 2020 - 12Z Sun Jan 03 2021 ...Complex systems to bring a heavy snow/ice threat from the central Plains to the Midwest/Great Lakes/Northeast mid and late week while also spreading heavy rain/convection across the South and the East... ...Overview and Guidance/Predictability Assessment... An amplified upper trough/low over the Southwest will lift northeastward across the Plains and may develop into a significant cyclone as it tracks into the Great Lakes Wed/Thu, with additional upstream energy approach also showing potential for subsequent/deepened low development Fri. The overall pattern is favorable, but system and stream phasing details are uncertain and that has led to larger than optimal run-run variances and focus changes. The WPC medium range product suite was mainly derived from a composite of the 12 UTC ECMWF/ECENS mean and small inputs from the more outlier/faster 18 UTC GFS/GEFS. This strategy maintains max WPC continuity in complex flow. Latest 00 UTC GFS/ECMWF/Canadian are trending to a less progressive Midwest main low Fri. This is certainly possible with a more separated southern stream. ...Weather/Threats Highlights... It remains likely that precipitation/convection will expand and intensify from the Plains eastward from midweek as the system intensifies and begins to draw deeper Gulf moisture and instability. The best potential for significant wrap-back snowfall, ice and high winds will extend from the central Plains to the upper Midwest/Great Lakes as depicted in the WPC Days 4-7 Winter Weather Outlook. Widespread areas along and well to the southeast of this axis will see heavy rainfall/strong to severe convection and some runoff issues in a favorable right entrance region of a potent upper jet from the south-central Plains to the mid-lower MS/OH Valleys, the South, then up the Appalachians/East Coast to finally close out 2020 and thankfully begin 2021. Wintry precip is possible from the mid-upper OH Valley/Appalachians to interior Northeast. Meanwhile, areas from the Pacific Northwest to northern CA will continue to see a return of rain/mountain snow this week in periodically enhanced fetch with approach of an uncertain but protracted series of Pacific systems. Highest totals will focus over favored coastal terrain and into the Cascades and Sierra. Additional activity will work over the Northwest/northern Rockies. There is an emerging signal to increase moisture feed into next weekend. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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