Extended Forecast Discussion NWS Weather Prediction Center College Park MD 138 AM EST Wed Dec 9 2020 Valid 12Z Sat Dec 12 2020 - 12Z Wed Dec 16 2020 ...General Weather Pattern... A shortwave pivoting around a broad upper level trough over the High Plains on Saturday will sustain a developing surface low over the Midwest states, and this low should remain well defined as it lifts northeastward towards the Great Lakes and eventually into Canada. There is also the potential for a wave of low pressure to develop along the trailing cold front over Gulf Coast region, although this remains uncertain. Meanwhile, two separate Pacific storm systems are forecast to affect Washington and Oregon with additional rain/mountain snow and gusty winds. The overall pattern is devoid of arctic air masses with most of the flow originating from the Pacific. ...Model Guidance Evaluation and Preferences... There are more model differences than are typically the case for this time period. This begins as early as Saturday across the north-central U.S. regarding the timing and amplitude of a second shortwave that will serve to reinforce the trough over the central U.S., with the 00Z GFS/GEFS mean initially slower across the Rockies. More noteworthy differences emerge by Sunday and especially into Monday with the 12Z/00Z ECMWF becoming much more amplified with the southern stream trough across the Gulf Coast region, leading to an impressive closed low, whereas the CMC/GFS/UKMET maintain an open trough that is more progressive. There has also been better run-to-run continuity with the past few GFS runs here, and the non-ECMWF consensus was preferred with below average confidence. Later in the forecast period next week, the CMC was stronger than the model consensus with the next trough crossing the Intermountain West with little in the way of ensemble support. Given the higher uncertainty, a greater percentage was placed on the EC and GEFS means along with some previous WPC continuity for the fronts and pressures forecast. ...Weather/Threats Summary... Temperatures are forecast to be slightly above normal across the eastern U.S. this weekend ahead of the cold front, with highs running well into the 50s and 60s for many areas. Given the lack of any arctic air masses behind the front, temperatures should return to near normal for most areas by early in the week, and perhaps slightly below normal for the south-central U.S. A corridor of showers and some thunderstorms is likely from the Gulf Coast to the Ohio Valley and Northeast states, and the progressive nature of this front should keep rainfall totals from becoming excessive. On the northwestern periphery of the precipitation, there will likely be wintry precipitation from Iowa to the northern Great Lakes with several inches of snowfall possible for some of these areas. It is worth noting that significant changes to the existing forecast are possible over the next few days across the Southeast U.S. and potentially the East Coast region, depending on the exact evolution of the next trough approaching from the Rockies early in the week. The Pacific Northwest will be in a rather unsettled pattern through the middle of next week with a couple of systems moving onshore and then across the Intermountain West. Coastal rain and mountain snow are expected this weekend with another round along with gusty winds early next week. The highest totals are expected with the weekend system with a few inches of rain possible across portions of northwestern California and western Oregon, and lighter totals with the second storm system. Hamrick 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: 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