Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1233 AM EST Thu Dec 10 2020 Valid 12Z Sun Dec 13 2020 - 12Z Thu Dec 17 2020 ...General Weather Pattern... Broad cyclonic flow aloft will be in place across most of the nation for the weekend and into early next week, with the main polar vortex remaining well to the north across northern Canada. A departing storm system over the Northeast U.S. this weekend will be followed by high pressure before a second low and cold front approaches by the middle of the week. There will be an active storm track across the northeast Pacific with strong surface lows impacting the Gulf of Alaska region and trailing cold fronts reaching the Pacific Northwest. With mid-upper level flow originating from the Pacific, no Arctic air masses are expected. ...Model Guidance Evaluation and Preferences... There has been an improvement in the model consensus for the Day 3-5 time period compared to this time 24 hours ago. The main change noted was with the 12Z ECMWF in comparison to its previous runs across the Gulf Coast region. Its solution is now more in line with the fairly consistent GFS in portraying a more progressive southern stream shortwave, instead of a potent closed low and strong storm system over the Southeast states. The 12Z CMC differs more from the ensemble means compared to the UKMET, GFS, and ECMWF early in the period, and shows a weaker and faster solution across the southern tier states. Later in the forecast period next week, the CMC is stronger than the model consensus with the next trough crossing the Intermountain West and then the southern Plains. There is even more model spread across the northeast Pacific and this leads to below average confidence for the middle of the week across the western U.S. Given the higher uncertainty, a greater percentage of the forecast blend was placed on the EC and GEFS means along with some previous WPC continuity for the fronts and pressures forecast beyond the weekend. ...Weather/Threats Summary... The system tracking across the Great Lakes and Northeast this weekend will likely bring some wintry precipitation across northern New England on Sunday, although the majority of the snow should be across Quebec. In the warm sector ahead of the trailing cold front, a corridor of showers are expected to extend from the Southeast to the Mid-Atlantic states. The progressive nature of the front should keep rainfall totals from becoming excessive. There is less confidence for precipitation coverage and amounts over the Southeast through Monday due to greater guidance spread for a possible trailing frontal wave. The Pacific Northwest and parts of northern California will be in a rather unsettled pattern through the middle of next week with a couple of systems moving onshore and then continuing across the Rockies. Expect coastal rain and mountain snow this weekend, along with another episode early-mid week. Periods of gusty winds will be possible as well, particularly near the coast. The greatest precipitation totals during this time should be across the coastal ranges and Cascades, and also extending into northwestern California. Snow showers will be likely for the northern and central Rockies. Temperatures will likely be 10-20 degrees above normal for the East Coast region on Sunday ahead of the cold front. The core of the coolest anomalies behind the cold front, up to 10 degrees below normal, should progress from the central Rockies/Plains to the southern Plains and Gulf Coast region. Otherwise temperatures are expected to be close to climatological averages. 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 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