Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Thu Dec 17 2020 Valid 12Z Sun Dec 20 2020 - 12Z Thu Dec 24 2020 ...Heavy precipitation pattern for the Northwest into the snowy northern Rockies... ...Overview and Model Guidance Evaluation/Preferences... It remains the case that 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 into next week. Models/ensembles show good agreement on the longwave pattern, but expectedly still significantly struggle days 3-7 with the individual shorter wavelengths. The pattern is set to amplify later next week, so predictability may benefit. The WPC medium range product suite was mainly derived from a blend of best-clustered guidance from the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECENS ensemble mean and the 01 UTC National Blend of Models early period. Aforementioned embedded system timing and strength variance suggested a quick switch to mainly ensemble weighting to maintain maximum WPC continuity. Latest 00 UTC guidance seems to offer minimal compelling reason to change this forecast strategy as embedded system run to run and model to model continuity is much less than stellar for many features. ...Weather/Threats Summary... Over the Northwest, rounds of Pacific moisture stream onshore with passage of a series of frontal systems. With this pattern, enhanced periods of coastal rain and elevation snows 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 holiday week along with quite windy conditions along and east of the Divide. Some models also brings a dynamic stripe of modest snows further east into the northern Plains and Upper Midwest/Great Lakes Tue-Wed. Moisture should be limited and ensembles offer less support. WPC guidance was conservative. but there is some favorable model continuity to monitor. 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 systems moves along the Southeast coast. Relative weakness of these systems may keep most precipitation along the coast and offshore and subsequent coastal waves have mixed signals in guidance. 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. There is a decent signal for flow amplification days 5-7 and upper trough progression from the Northwest to the east-central U.S. may increase moisture inflow and modest precipitation coverage into Christmas Eve. There is some risk for snow on the northern periphery of this precipitation shield from the OH Valley/Great Lakes to the Appalachians and Northeast. Temperatures will generally be near to above normal (perhaps by 10-20 degrees over the Plains) through early next week given the Pacific-dominated flow. By early next week, cooler than normal temperatures may filter out through the Rockies/Great Basin in the wake of a cold front. 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