Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Tue Dec 15 2020 Valid 12Z Fri Dec 18 2020 - 12Z Tue Dec 22 2020 ...Heavy precipitation pattern for the Northwest and snowy northern Rockies... ...Model Guidance Evaluation and Preferences... Model and ensemble solutions are fairly well-clustered in a pattern with seemingly above average predictability, at least through the weekend. The WPC medium range product suite was accordingly manually derived from a large composite blend of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/Canadian/UKMET/ECENS mean and the 01 UTC National Blend of Models. Quickly growing forecast spread with small-mid scale embedded features by early next week forced most blend weighting from the models this week, to the more compatible ensemble means and NBM into Day 6/7. This blend forecast strategy maintains good WPC continuity. ...Weather/Threats Summary... 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 to the northern Rockies. Downstream propagation of energies from the West will induce Gulf of Mexico moisture to return northward through the east-central U.S. by the weekend along/ahead of emerging frontal systems near the Gulf Coast. An expanding precipitation shield should spread up through the Eastern Seaboard by early next week 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. There is some potential for a stronger southern stream shortwave and wetter surface system than depicted in my progs that were conservatively constructed given the large model forecast spread issue through the 18 UTC guidance cycle, but there is now a growing 00 UTC guidance signal to monitor. 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