Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EST Tue Feb 09 2021 Valid 12Z Fri Feb 12 2021 - 12Z Tue Feb 16 2021 ...Arctic air lingers over the Plains and the Midwest... ...Significant late week upslope/mountain snow possible in the northern and central Rockies... ...A couple of winter weather events possible for the Mid-Atlantic to Northeast late week and weekend... ...Pattern Overview and Guidance/Predictability Assessment with Weather Highlights... A large portion of the northern and central U.S. will remain under the grip of an arctic air mass through much of the medium-range period. Global models continue to indicate that a polar vortex centered in the Canadian prairies will remain the dominant synoptic system. The ECMWF and to some degree the CMC, agree that the vortex will begin to weaken and slide eastward across the Great Lakes by this weekend. On the other hand, the GFS tends to grab the upper-level energy coming across the Pacific Northwest and pull it around the south side of the polar vortex, resulting in frontal wave development in the eastern U.S. during the weekend. The ECMWF/CMC solution is preferred given better ensemble mean support as well as better agreement with WPC continuity. This setup favors a continuation of frigid conditions over the northern Plains and upper Midwest as a couple of wintry weather events are possible late this week and later in the weekend from the Mid-Atlantic to Northeast. The specifics are far from certain at this point. Over the northern and central Rockies into the Intermountain region, models continue to signal for increasing amounts of upslope/mountain snow under a tight pressure gradient behind the arctic front late this week into the weekend. Meanwhile, three Pacific systems are forecast to move into the West Coast during the medium range, bringing periods of mountain snows and coastal rains across the Pacific Northwest and down into northern California with reasonable model spread. Along a wavy/stationary boundary in the Gulf, models are in good agreement for a period of enhanced rainfall to overspread the Gulf Coast States with likely inland focuses for the interior Southeast. By the weekend into early next week, there is decent model consensus for an upper-level shortwave to dip across the Southwest and then move out into the southern Plains to bring a swath of wintry weather across the southern Rockies and the south-central U.S. Temperature wise, the frigid air will continue over the Northern Plains, Northern Rockies, and Upper Midwest for much of the period where anomalies of 30-45F are forecast across the middle of the country. This is expected to slowly overspread, but moderate somewhat, across much of the country by the end of the forecast period. Best chance for record low temperatures will be up and down the Mississippi Valley as well as the central and southern Plains on Sunday. The WPC medium-range forecast package was derived from a composite blend of the 12Z ECMWF/EC mean, the 18Z GFS/GEFS, together with a smaller contribution from the 12Z CMC/CMC mean and the 01Z NBM. Mostly ensemble means were used on Days 6 and 7 given the increasing model uncertainties. Kong Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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