Extended Forecast Discussion NWS Weather Prediction Center College Park MD 340 PM EST Tue Dec 15 2020 Valid 12Z Fri Dec 18 2020 - 12Z Tue Dec 22 2020 ...Heavy precipitation pattern for the Northwest into the northern Rockies... ...Overview and Model Guidance Evaluation/Preferences... Upper pattern over the weekend into next week will feature quasi-zonal progressive flow. Upper low over NW Canada/Nunavut and ridging west of and into California is forecast to direct 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 western North America (British Columbia to Oregon) next week. The models/ensembles show good agreement on the longwave pattern but expectedly struggle with the individual shorter wavelengths. Ensemble trends over the past few days show the largest spread squarely over the Pacific Northwest with no significant decrease in spread over the past two days, suggesting a low lead time on details perhaps to 3 days. A multi-model/ensemble blend was utilized to minimize run-to-run changes, with a preference toward the 00Z ECMWF/Canadian for the eastern portion of the lower 48 but a blended starting point in the West. There, faster GFS solutions were not totally discounted given the pattern. By early next week, ensembles suggest another trough will take aim at the Pacific Northwest with the possibility of coastal cyclogenesis off the Southeast coast. Confidence was low in any given model solution, but ensemble clustering maintained a somewhat weak system lifting northeastward along the coast just off New England into Atlantic Canada. Pacific system should move swiftly into the Rockies as the pattern remains progressive. Trended toward an ensemble consensus to mitigate future timing/track changes of the individual systems. ...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 medium range period in Washington and Oregon, focused this weekend and again early next week. Greatest uncertainty lied over southwestern Oregon into northern California with respect to amounts, depending on the onshore flow evolution and orientation. Highest totals will be in the favored upslope/higher elevations of the Olympics, Coastal Ranges, and Cascades. Enhanced snows will spread inland to the northern Rockies. Daily totals could exceed 1-4 inches with multi-day totals at least twice that. Lower elevations may see 1-3 inches over the period west of the Cascades but less to the east. System exiting the southern Rockies Friday will encourage an increase of moisture into the lower Mississippi 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 evening. Uncertainty increases in affects next week as the system may remain weak and not pull in much moisture into the Mid-Atlantic and Northeast. However, with a subsequent system over the High Plains, there may be an opportunity for some development with more appreciable, though perhaps modest at best, rainfall along the coast and some snow inland. Depicted an offshore precipitation shield that will be modified over the next few days. Northern/northeastern Maine has a higher potential for snowfall by the end of the period but with high uncertainty. Fracasso 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 precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Mon, Dec 19-Dec 21. - Flooding possible across portions of the Pacific Northwest. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Mon, Dec 19-Dec 21. - Heavy snow across portions of the Northern/Central Rockies, Sun-Mon, Dec 20-Dec 21. - High winds across portions of the Northern/Central Rockies and the Northern/Central Plains, Sat-Sun, Dec 19-Dec 20. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat, Dec 19. 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