Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Feb 08 2021 Valid 12Z Thu Feb 11 2021 - 12Z Mon Feb 15 2021 ...Arctic air lingers over the Plains and the Midwest... ...Possible Mid-Atlantic to Northeast U.S. winter storm late this week as unsettled weather reaches the West Coast... ...Pattern Overview and Guidance/Predictability Assessment with Weather/Threats Highlights... Latest model guidance continues to show that an arctic air mass will dominate the northern and central portions of the country into this weekend under the influence of a slow-moving polar vortex centered over the Canadian prairies. There is general agreement that the vortex will slowly slide eastward and weaken/open up with time. Meanwhile, models continue to advertise a significant influx of Gulf moisture across the Deep South into the Southeast mid to late this week to the north of the polar front. A swath of wintry precipitation remains in the forecast from the Ohio Valley to portions of the Northeast mainly on Thursday and Friday as a couple of frontal waves develop well to the south. Latest model consensus has trended slightly toward less amplitude and a more offshore storm track. This is in response to a model trend of rotating less energy to the south of the polar vortex across the Great Lakes late this week--a change that is most noticeable in the GFS, which now agrees much better with the ECMWF regarding this feature. Meanwhile, models have continued to slow down the southward intrusion of the arctic air mass through the mountainous terrain of the western U.S., with a better signal for upslope snows to linger over the northern to central Rockies. A couple of weakening Pacific systems will then push onshore into the West Coast with mountain snows and rain for the lower elevations through the latter half of the week into next Monday. Much of the precipitation should fall along and to the west of the Cascades and down into California. Some of the moisture will spread into the Great Basin and interior Southwest, reaching the central to southern Rockies by the weekend. Elsewhere, upper-level energy ejecting from the central Rockies could bring a swath of light snow across the north-central Plains Thursday into Friday. By next Monday, Gulf moisture could be returning into the southern Plains with some wintry weather possible over the central Plains. 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. More of the ensemble means were used on Days 6 and 7 as well given the increasing model uncertainties. The GFS and the CMC were eliminated on these two days due to spurious cyclogenesis on the East and West Coasts respectively. 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