Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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... The medium range period will begin with a northern stream upper trough moving through New England on Tuesday. Meanwhile, an amplified upper trough/low over the Southwest will lift northeastward across the Plains and will likely develop into a major cyclone as it tracks into the Great Lakes by midweek. Since yesterday, models have shown a trend toward delaying the onset of cyclogenesis over the central Plains next Wednesday. On the other hand, the GFS remains to be the fastest guidance in terms of taking the cyclone across the Great Lakes Wednesday night, so much so that it disagrees significantly with the 06Z GEFS and 00Z EC mean by next Thursday. The CMC offers the slowest solution with a deep cyclone moving into the Great Lakes not until Friday. Meanwhile, the GFS develops another deep cyclone over the Mid-Atlantic in the wake of the previous storm. Overall, there was more inherent uncertainty in this forecast package but preference was given to the slower guidance because of better ensemble guidance support. The WPC medium range product suite was derived from a multi-model/ensemble composite of the 00Z ECMWF/00Z EC mean, the 06Z GEFS, the 00Z CMC, the 06Z GFS early in the period, and the 00Z NAEFS. This offers a slower progression of the midweek cyclone across the Plains to the Great Lakes compared with previous WPC forecasts, which is supported by the latest (12Z) ECMWF. ...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 high 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 as depicted in the WPC Days 4-7 Winter Weather Outlook. Again, the GFS becomes a fast outlier with the main system by midweek. The latest WPC guidance shows a slower progression of the main cyclone as well as the associated cold front later moving into the East Coast. 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. Kong/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