Extended Forecast Discussion NWS Weather Prediction Center College Park MD 353 PM 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 Pacific 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 Sunday, suggesting a low lead time on details. However, spread has decreased over the past few ensemble runs. That area of uncertainty translates to the east for early next week, suggesting lower predictability there as a northern and coastal system attempt to deepen. Trends suggest the northern stream system will be the dominant one through the Great Lakes. By next Tue-Wed, yet another Pacific system will push through the Rockies and toward the Upper Midwest. A multi-model/ensemble blend with best-clustered guidance from the 06Z GFS/GEFS mean and 00Z ECMWF/ECENS/Canadian/UKMET was utilized to minimize run-to-run changes, with a majority deterministic guidance for the Sat-Mon period focused nearer to the ECMWF. Trended toward majority ensemble weighting (preference toward the ECMWF ensembles) maintained good WPC continuity and acted to mitigate timing/track changes of individual systems. ...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 to be in the favored upslope elevations of the Olympics, Coastal Ranges, and Cascades with multi-inch totals likely. Enhanced snows will spread inland to the northern Rockies along with quite windy conditions along and east of the Divide. 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 along the Southeast coast Sunday. Relative weakness of this system may keep most precipitation along the coast and offshore. Trailing series of frontal waves should be fairly moisture-limited, concentrating along the U.S.-Canadian border. Some lake enhancement is possible to the east of the Lakes. Temperatures will generally be near to above normal (perhaps by 10-20 degrees over the Plains) through the period given the Pacific-dominated flow. By early next week, cooler than normal temperatures may filter through the Rockies/Great Basin in the wake of a cold front. Fracasso/Schichtel Hazards: - Heavy precipitation across portions of the Pacific Northwest, Sat-Mon, Dec 19-Dec 21. - Heavy snow across portions of the northern Rockies, Sat-Mon, Dec 19-Dec 21. - Flooding possible across portions of the coastal plain in northwestern Washington. - High winds across portions of the northern Rockies, Sat-Mon, Dec 19-Dec 21. 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