Extended Forecast Discussion NWS Weather Prediction Center College Park MD 146 AM EST Sat Dec 26 2020 Valid 12Z Tue Dec 29 2020 - 12Z Sat Jan 02 2021 ...Significant low pressure system to bring a heavy snow/ice threat from south-central parts of the Great Basin/Rockies/Plains to the Midwest/Great Lakes/Northeast early to middle of next week before spreading heavy rain/convection across the South and into the East Coast late next week... ...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 through New England by Tue. Meanwhile, an amplified upper trough/low over the Southwest will lift northeastward across the Plains and will likely develop into a major cyclone and work into the Great Lakes by midweek. Models continue to indicate that a sharp front will extend south from the cyclone and sweep across the central and eastern U.S. midweek into New Year's Day. There are manageable run-to-run fluctuations in frontal speed and cyclone intensity in large part due to stream phasing differences, but the cyclone track remains rather consistent. 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/ECENS mean and the 01 UTC National Blend of Models. Shifted blend focus from the models Tue/Wed increasingly to the ensembles later next week consistent with gradually growing forecast spread in an overall pattern with above normal mid-larger scale predictability. This solution maintains good WPC continuity. Latest 00 UTC model and ensemble guidance still seems in line with this overall solution, but the short range will reveal focus details. ...Weather/Threats Summary... Tue exit of a deepened lead low from the Northeast to the Canadian Maritimes will offer mostly light precipitation and some brisk/windy conditions. The main system of interest this period will bring rain and elevation snows over the south-central Great Basin/Rockies into Tue. Amounts will generally be moderate, but with some localized enhancement over windward terrain and with deformation. Precipitation/convection will expand and intensify from the Plains eastward Tue/Wed 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 this period as depicted in the WPC Days 4-7 Winter Weather Outlook. The GFS becomes a fast outlier compared to WPC progs with the main system by midweek, but the GFSp trends slower, as has the 00 UTC ECMWF. 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 highly 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. Some wintry precip is then possible from the mid-upper OH Valley/Appalachians to the interior Northeast. 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