Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 PM 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... As an energetic southern stream upper trough progresses 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, rounds of precipitation are forecast in the Northwest with a progressive pattern and persistent Pacific moisture inflow. ...Model Guidance Evaluation and Preferences... Model guidance continues to be agreeable with indicating the potent nor'easter and cluster fairly well with the nor'easter low track, bolstering forecast confidence. Thus through Wed/Thu, a multi-model deterministic blend of the 00Z ECMWF, UKMET, and CMC and the 06Z GFS was able to be used for the medium range forecast blend, and the ensemble means were in good alignment as well. This compromise solution seemed best for now, but even seemingly minor low track shifts could cause notable sensible weather differences. The mid/upper trough associated with this feature should shift offshore by Fri. Meanwhile, additional upper troughing/energy is forecast to come into the West Coast Thu and move eastward through the central CONUS through the weekend. While model guidance has reasonable agreement with this longwave pattern, detail differences remain with the axis and orientation of the trough and embedded energy. Gradually weighted the GEFS and ECMWF ensemble means more as the medium range period progressed to minimize the individual model differences. ...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-level 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. 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, so continue to monitor updated forecasts as this system shifts into the short range timeframe in the coming days. Across the northwestern U.S., rounds of Pacific moisture are expected to stream onshore as a train of frontal systems approach. With this pattern, periods of coastal rain and higher elevation snow are forecast through next weekend 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, with lesser snow amounts in the Wasatch and Central Rockies. Elsewhere, generally dry conditions should prevail toward the end of the workweek. Then by next weekend, confidence continues to increase that Gulf of Mexico moisture will return to south-central and southeastern portions of the U.S. ahead of the upper trough and bring rain to those regions. Tate 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