Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1106 AM EST Wed Dec 23 2020 Valid 12Z Sat Dec 26 2020 - 12Z Wed Dec 30 2020 ...Anomalous/potentially snowy and wet system expected in the Plains/Midwest/Mississippi Valley mid next week... ...Overview and Model Guidance Evaluation/Preferences... Fairly progressive flow aloft will contain a series of features with varying amplitude, bringing one or more episodes of active weather to parts of the country during the Saturday-Wednesday period. The vigorous upper trough should lift through the Northeast on Saturday. An upstream shortwave will bring low pressure into the central U.S. by Sunday. Complexity of dynamics affecting the system crossing the eastern half of the country early in the week has led to more spread and variability over recent days. Latest adjustments have been toward somewhat stronger northern stream involvement (apparently caused by a stronger system moving through the Southwest), favoring a surface low track into the Great Lakes by early Monday and then into southeastern Canada/New England with the potential for redevelopment near the New England coast followed by an eastward or northeastward track. The next system will amplify toward California Sunday-Monday and continue inland thereafter. Another system approaches the Pacific Northwest late next week -- the 00z ECMWF was a southerly, somewhat quick, and very wet outlier here. While the guidance shows reasonable agreement, day to day changes in the Northeast and some slight acceleration in the general progression of systems leaves lingering question marks regarding stability and certainty. A compromise of the 00z Canadian, 00z UKMET, 06z GFS, and 00z ECMWF was preferred early on for pressures, winds, fronts, and the precipitation forecasts before greater inclusion of the 00z ECMWF ensemble mean and 00z NAEFS ensemble mean was utilized for early to mid next week. The remainder of the grids were a more even deterministic/ensemble mean mix through the period. ...Weather/Threats Summary... The Northeast will trend drier this weekend though with some lingering lake effect snow likely continuing into Saturday. Best potential for meaningful accumulations will be downwind from Lake Ontario. The first system to affect the West will bring moderate amounts of rain and higher elevation snow this weekend to areas from the Pacific Northwest into northern and perhaps central California, with lesser totals reaching into the northern half of the Rockies. The upper level system that continues eastward will likely combine with Canadian flow to support central/northeastern U.S. low pressure from Sunday onward, bringing generally moderate precipitation from the Ohio Valley into the Northeast with lighter/more scattered activity farther south. By Tuesday, enough uncertainty exists to reign in confidence for the precise precipitation/wind details. Any snow should be over northern latitudes and into the central Appalachians. Overall confidence was average for the system forecast to head into California. Flow aloft may be favorable for terrain enhancement of precipitation for a period of time over southern California. During Monday-Tuesday expect rain and higher elevation snow to spread across the southern two-thirds of the West. This should be welcome precipitation given the relatively dry conditions that have persisted over portions of this region so far this season. By late Tuesday-Wednesday, precipitation should develop and expand over the central U.S. and Midwest with a range of precipitation types, including potentially heavy rain in the warm sector and heavy snow in the cold sector. System progression will bring some variability for temperatures, with the general trend for a majority of the lower 48 to see near to somewhat below normal highs by the early/middle part of next week. The central/southern Plains should see highs 10-15F above normal during the weekend followed by a cooling trend behind a cold front. Areas near the East Coast will start the weekend on the chilly side with the most extreme anomalies for highs likely to be over Florida on Saturday (15F or more below normal at some locations). The eastern U.S. will moderate after Saturday. The Upper Midwest will see some highs at least 10F below average around Sunday-Monday. Expect highs to be mostly 5-10F below normal within the cooler air forecast to spread into/across the West and eventually into the Plains. Roth/Rausch 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