Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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. 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 intrusions are expected into the U.S. Meanwhile, a departing storm system over the Northeast U.S. this weekend will likely be followed by an amplifying upper trough over the mid-section of the country. The northern stream and southern energies could phase to trigger significant cyclogenesis near the Mid-Atlantic coast by next Wednesday into Thursday. ...Model Guidance Evaluation and Preferences... There has been a continued improvement in the model consensus compared to this time yesterday, especially for next Sunday into Monday from the Southeast to the Mid-Atlantic region. The ECMWF has continued to depict a solution similar to 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 CMC offers a similar pattern evolution for the medium range as well. Later in the forecast period, global models have been trending toward a closer interaction between a northern stream trough dipping into the northern High Plains and the southern jet stream near the Gulf Coast since yesterday. This has resulted in fairly decent model consensus on the formation of what appears to be a rather robust cyclone near the Mid-Atlantic coast Wednesday into Thursday. The WPC medium range sea-level pressure fields were derived based on an even blend of the 06Z GFS/GEFS and the 00Z ECMWF/EC ensemble mean. This blend smooths out the uncertainty related to the active cyclone tracks off the coast of the Pacific Northwest and a deep cyclone off the New England coast next Thursday. Decent WPC continuity was maintained. ...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 is expected to extend from the Southeast to the Mid-Atlantic states. The progressive nature of the front should keep rainfall totals from becoming excessive. Confidence for precipitation coverage and amounts over the Southeast through Monday has increased greater guidance agreement for the passage of a developing frontal wave. By late next Wednesday into Thursday, there is an increasing potential for significant wintry precipitation across the Northeast. 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 through the northern to central 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 the 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 trailing cold front associated with the New England storm. 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 not expected to depart too far from climatological averages. Kong/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