Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 AM EST Wed Dec 16 2020 Valid 12Z Sat Dec 19 2020 - 12Z Wed Dec 23 2020 ...Heavy precipitation pattern for the Northwest into the snowy northern Rockies... ...Overview and Model Guidance Evaluation/Preferences... The upper pattern over the weekend into next week will feature quasi-zonal progressive flow that will channel Pacific moisture into the Northwest via several embedded disturbances, contributing to a heavy rainfall threat over the multi-day period. Strong 200+ kt jet off Japan will aide in the split flow over the central Pacific, with the northern branch directed into the Northwest next week. Models/ensembles show good agreement on the longwave pattern, but expectedly struggle with the individual shorter wavelengths. Ensembles still show the largest forecast spread squarely over the Pacific Northwest, suggesting a low lead time on details to perhaps 3-4 days. A multi-model/ensemble blend with best clustered guidance from the 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble means was utilized to minimize run-to-run changes, with a preference transitioning from the models day 3/4 toward the ensemble means by days 6/7. The trend toward ensembles maintains better WPC continuity and acts to mitigate timing/track changes of individual systems. Latest 00 UTC guidance seems in line with the aforementioned forecast strategy. By early next week, guidance suggests another dynamic trough will take aim at the Pacific Northwest. Strong upstream ridge amplification now in most guidance offers a growing signal for trough amplification over the Northwest, then through the Rockies to the central U.S. midweek. This seems best shown by the ECMWF and ensembles and may lead to another return of Gulf moisture. ...Weather/Threats Summary... Over the Northwest, 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 through the medium range period in Washington and Oregon, focusing this weekend and again early next week. Highest totals are expected be in the favored upslope elevations of the Olympics, Coastal Ranges, and Cascades. Enhanced snows will spread inland to the northern Rockies. Amplified upper level trough progression will encourage an increase of moisture into the lower MS Valley as the cold front and area of low pressure slip through the northern Gulf this weekend. Rainfall will expand eastward as the system moves toward the Southeast coast Sunday. Uncertainty increases in affects next week, but a potential trailing series of frontal waves at this point seem relatively weak and do not pull in much moisture into the Mid-Atlantic and Northeast. Expect modest rainfall along the coast and some snow inland. Northern/northeastern Maine has a higher potential for snowfall late period, but with high supporting low system uncertainty. 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