Extended Forecast Discussion NWS Weather Prediction Center College Park MD 339 PM 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 were fairly well-clustered 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 06 UTC GFS/GEFS mean, 00 UTC ECMWF/ECENS mean and the National Blend of Models. This maintains very good WPC continuity, with a slightly faster eastward progression of the frontal system across the Great Lakes/Ohio Valley during the weekend. Latest 12 UTC ECMWF remains in line with the overall pattern evolution. ...Weather/Threats Summary... A significant nor'easter will likely reach peak intensity Thursday morning with heavy snow impacting parts of central New England while rain and mixed precipitation move across southern New England. Precipitation will likely taper off from west to east during the day on Thursday as the cyclone moves further off the coast. High winds and rough seas near the coast will gradually moderate as well. 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. Model guidance continues to signal that downstream propagation of energies from the West may induce Gulf of Mexico moisture to return northward through the east-central U.S. by next weekend along/ahead of emerging frontal systems near the Gulf Coast. The modest but expanding precipitation shield may then spread through the Eastern Seaboard late period and offer wintry potential on the colder northern periphery over the northern Appalachians/Mid-Atlantic and Northeast where activity that may be enhanced by modest coastal cyclogenesis Sunday into Monday. Kong/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 Hazards: - Heavy snow across portions of the Northeast, Thu, Dec 17. - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Mon, Dec 19-Dec 21. - Heavy snow across portions of the Northern/Central Rockies and the Northern/Central Great Basin, Thu, Dec 17 and Sat-Mon, Dec 19-Dec 21. - High winds across portions of the Central Great Basin, the Northern/Central Rockies, and the Northern/Central Plains, Sat-Sun, Dec 19-Dec 20. 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