Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Dec 25 2020 Valid 12Z Mon Dec 28 2020 - 12Z Fri Jan 01 2021 ...Significant system to bring a heavy snow/ice threat from south-central parts of the Great Basin/Rockies/Plains to the Midwest/Great Lakes/Northeast next week. There is a heavy rain/convection threat for The South and East... ...Overview and Model Guidance Evaluation/Preferences... Moderately progressive flow will carry a couple of significant upper troughs with impactful weather across the nation next week. A northern stream upper trough will move from the Great Lakes through New England early next week. Meanwhile, an amplified upper trough/closed low moving onshore over CA and the Southwest will lift over the Plains/MS Valley/Midwest by midweek with major storm genesis, exiting the East into eastern Canada New Year's Day. The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 18 UTC GFS/GEFS, the 12 UTC ECMWF/UKMET/ECENS mean and the 01 UTC National Blend of Models Mon-Wed. Growing forecast spread prompted to gradual transition to the ensemble means and NBM later next week. This solution maintains good WPC continuity. Latest 00 UTC model and ensemble guidance remains overall in line, but locally focusing smaller scale details and interactions will evolve into short time scales. ...Weather/Threats Summary... An early next week exit of a deepened lead low from the Great Lakes/Northeast to the Canadian Maritime will offer mostly light precipitation. This system should also bring a period of brisk/windy conditions along with a brief episode of lake effect snow in the wake of system passage. The main system of interest this period will bring rain and elevation snows down across CA into Mon, with precipitation extending over the south-central Great Basin/Rockies early next week. Amounts will generally be moderate, but with some localized enhancement over windward terrain and with deformation. Precipitation/convection will then expand and intensify from the Plains eastward by Tue/midweek as the system develops and begins to draw deeper Gulf moisture and instability. The best potential for significant snowfall/some ice and high winds extends from the south-central Great Basin/Rockies to the south-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 may see heavy rainfall/strong to severe convection and some runoff issues in a highly favorable right entrance region of a potent upper jet. Some wintry precip is also possible from the upper OH Valley/Appalachians through the Northeast to finally and thankfully end year 2020. Meanwhile, areas from the Pacific Northwest to northern CA will see a return of rain/mountain snow next week in periodically enhanced fetch with approach of an uncertain series of Pacific systems. Highest totals will focus over favored coastal terrain and into the Cascades and Sierra. Somewhat lighter activity may work over the Northwest into the northern Rockies. 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