Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EST Sat Dec 12 2020 Valid 12Z Tue Dec 15 2020 - 12Z Sat Dec 19 2020 ...Midweek Nor'easter to focus Heavy Winter Threat for Mid-Atlantic and Northeast... ...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 Tue/Wed. The latest 00 UTC GFS/ECMWF fit this mold for this time window. Opted to mostly favor a GEFS/ECMWF ensemble and NBM composite later through Day7 to maintain WPC continuity in a pattern with increasing embedded system forecast spread and uncertainty. ...Weather/Threats Summary... The Pacific Northwest and adjacent northern California will be the focus of where Pacific moisture streams onshore through much of next week with a couple of systems moving onshore and then across the northern to central Rockies. Expect coastal rain and mountain snows with each wave. 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, the Cascades, and northwestern California. Enhanced snows will also spread inland across the Intermountain West, with the ranges of Idaho/northwest MT, and northwest WY having the highest probabilities in the WPC Winter Weather Outlook for Days 4-7. System predictability seems very low at long time frames, but the progressive system pattern should prevail. The guidance signal is growing in support for an amplified upper trough to emerge into the south-central U.S. Tue then progress through the east-central states by Wed and off the East Coast by Thu. There is a risk for a modest but disruptive swath of snow/ice from the cooled southern Plains northeastward to the OH Valley. Frontal system progression and organization seems to lead to development of a deepening coastal storm off the Mid-Atlantic and Northeast Wed into Thu. WPC progs low positions are closer to the strengths of the models, but closer to the position of the ECMWF ensemble mean (slightly faster than the ECMWF and slower than the GFS). This detail is significant considering a deep wedge of cold air will be settled down from the Northeast to the Mid-Atlantic. This offers a significant threat of heavy snow/ice from the Appalachians through the Upper OH Valley/Interior Mid-Atlantic to the Northeast Wed into Thu as depicted on the WPC Winter Weather Outlook. The main Metro Corridor are now within the latest risk probabilities, but not the max axis. 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