Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Mon Dec 14 2020 Valid 12Z Thu Dec 17 2020 - 12Z Mon Dec 21 2020 ...Nor'easter to track off New England Thursday... ...Heavy precipitation pattern for the Northwest this week... ...Model Guidance Evaluation and Preferences... Model and ensemble solutions seem fairly well clustered, especially days 3-5 (Thu-Sat), in a pattern with seemingly overall above average predictability. The WPC medium range product suite was accordingly manually derived from a composite blend of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/Canadian/ECENS mean and the 01 UTC National Blend of Models. This maintains great WPC continuity. Latest 00 UTC guidance remains in line. ...Weather/Threats Summary... A significant nor'easter will reach Canadian Maritimes waters Thu as lingering high winds and heavy snows wrap-back into the Northeast. Across the northwestern U.S., rounds of Pacific moisture are expected to stream onshore with passage of a series of frontal systems. With this pattern, enhanced periods of coastal rain and elevation snow are forecast for the coming week in the Pacific Northwest and northern California/Sierra, with highest totals in the favored higher elevations of the Coastal Range and Cascades. Enhanced snows will spread inland across the Intermountain West and the Northern Rockies where the Wind River Mountains/Tetons and Wasatch/Uinta mountains could see notable snow totals. A growing guidance signal shows that the downstream propagation of energies from the West may induce Gulf of Mexico moisture return northward through the east-central U.S. by next weekend along/ahead of emerging frontal systems. The modest but expanding precipitation shield may then spread through the Eastern Seaboard late period and offer wintry potential on the more cooled northern periphery over the northern Appalachians/Mid-Atlantic and Northeast where activity that may be enhanced by uncertain coastal low genesis. 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