Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Fri Dec 11 2020 Valid 12Z Mon Dec 14 2020 - 12Z Fri Dec 18 2020 ...Wintry Weather Threats... ...Model Guidance Evaluation and Preferences... The WPC medium range product suite was manually derived from a composite blend of reasonably well clustered guidance from the 18 UTC GFS/GEFS, 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC National Blend of Models Mon/Tue. Leaned this composite toward the ensembles and ECMWF Wed-Fri to maintain better WPC continuity in a pattern with increasing embedded system forecast spread and uncertainty. Later 00 UTC guidance remains varied at these longer time frames with minimal clarifying signals yet. ...Weather/Threats Summary... Lead southern stream upper trough progression out through the Southeast Monday is likely to spawn a frontal wave along a trailing East Coast cold front. Expect a swath of moderate precipitation from the southern Appalachians to the Carolinas and southern VA and vertical temperature profiles seem to support some snow/ice on the northwest periphery of the progressive precipitation shield. The Pacific Northwest and parts of northern California will be in a rather unsettled pattern through the middle of next week with a couple of systems moving onshore and then through the northern to central Rockies. Expect coastal rain and mountain snow this weekend, along with another episode early-midweek. Periods of gusty winds will be possible as well, particularly near the coast and in channeling terrain. Greatest precipitation totals during this time should be across the Coastal Ranges and the Cascades, extending into northwestern California. Enhanced snows will also spread inland across the Intermountain West to the northern and central Rockies as depicted on the WPC Winter Weather Outlook. Upper support/tilt and amplitude/timing become more uncertain with emergence downstream over the South then up the East as a series of fronts and lows work into the region. Midweek system ejections and a threat of coastal low genesis though remains interesting to monitor/prepare considering a possible wedge of cold air may by then be settled down from the Northeast to the Mid-Atlantic. This offers a threat of heavy snow/ice from the Appalachians through the interior Mid-Atlantic through the Northeast Wed into Thu as depicted on the WPC Winter Weather Outlook. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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