Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Dec 13 2020 Valid 12Z Wed Dec 16 2020 - 12Z Sun Dec 20 2020 ...Midweek nor'easter to focus significant winter threat for the Appalachians/Mid-Atlantic and Northeast... ...Overview... An energetic southern stream upper trough will progress into the East Wed/Thu, combining surface lows are expected to deepen near the Mid-Atlantic coast and form a nor'easter likely to spread significant wintry weather across portions of the Central Appalachians, Mid-Atlantic, and Northeast Wed and lingering in New England Thu. Meanwhile, precipitation persists in the Northwest with a progressive pattern and persistent Pacific moisture inflow. ...Model Guidance Evaluation and Preferences... Guidance clustering has improved through medium range time scales, bolstering forecast confidence. The WPC medium range product suite was manually derived from now well clustered guidance from the GFS/ECMWF/Canadian/UKMET for Wed/Thu. The GEFS and ECMWF ensemble means and the 01 UTC National Blend of Models were also in line. Despite better guidance agreement into Day 5-7 compared to the last few days, enough uncertainty remains for promotion of a more GEFS/ECMWF ensemble and NBM composite through this period to best maintain WPC continuity. Model information was still used, but with smaller blend input weightings. ...Weather/Threats Summary... Ejecting low system energies from the central U.S. remain slated to combine and develop into a nor'easter impacting the Eastern Seaboard Wed into Thu. Significant heavy snow is likely to occur across parts of the Central Appalachians, Mid-Atlantic, and Northeast, with potential for ice and high winds in some areas as well as moisture feeds into a lower atmospheric cold wedge. The highest probabilities for heavy snow appear to be just west of the major cities along I-95, but the large cities certainly could see impactful wintry weather as well. And as mentioned, even small shifts in the low track could cause sizable differences in the precipitation type at a particular place and with how far west over land the precipitation shield spreads. Across the northwestern U.S., rounds of Pacific moisture are expected to stream onshore as frontal systems approach. With this, periods of coastal rain and higher elevation snow are forecast through the end of the week in the Pacific Northwest and adjacent northern California, with higher totals in the favored higher elevations of the Coastal Range and Cascades. Snow will spread inland into the Intermountain West as well, and the Northern Rockies and Wind River Mountains/Tetons could see notable snow totals. Elsewhere, generally dry conditions should prevail toward the end of the workweek; then there is an increasing chance of Gulf of Mexico moisture return causing the possibility for rain up through the South next weekend, and confidence in this scenario is slowly increasing. 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