Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Dec 25 2020 Valid 12Z Mon Dec 28 2020 - 12Z Fri Jan 01 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 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 northeastward across the Plains and will likely develop into a major cyclone over the Great Lakes by midweek. Models continue to indicate that a sharp front will likely extend south from the cyclone center and sweep across the entire central and eastern U.S. from midweek into New Year's Day. There continues to be run-to-run fluctuations in the speed of progression of the front as well as the intensity of the cyclone. The cyclone forecast track remains rather consistent with each new model run even though its forward speed continues to fluctuate. The GFS remains the fastest guidance regarding the cyclone track whereas the CMC is the slowest. The ECMWF is somewhere in between. Given the prevalence of a highly amplified synoptic pattern, a slower solution similar to the ECMWF is preferred for the longer range forecasts. Therefore the WPC medium-range package incorporated a higher percentage from the 00Z ECMWF and 00Z EC mean for Days 6 and 7. An even blend of the 06Z GFS and 06Z GEFS together with the 00Z ECMWF/EC mean were used for Days 3 to 5. This solution maintains good WPC continuity. ...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. In the wake of this system, a period of brisk/windy conditions across the Great Lakes should bring a brief episode of lake effect snow. The main system of interest this period will bring rain and higher elevation snows down across CA into Mon, with precipitation extending over the south-central Great Basin into the central 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 Tuesday into midweek as the system intensifies and begins to draw deeper Gulf moisture and instability. The best potential for significant snowfall, some ice and possibly some high winds will likely 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 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 behind what appears to be another sharp cold front moving through the eastern U.S. later next week. A swath of heavy rain can be expected to sweep across the interior South on Wednesday before streaming up the East Coast once again to close out 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. 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