Extended Forecast Discussion NWS Weather Prediction Center College Park MD 110 PM EST Sun Feb 07 2021 Valid 12Z Wed Feb 10 2021 - 12Z Sun Feb 14 2021 ...Arctic air lingers over the Plains and the Midwest... ...Possible Mid-Atlantic to Northeast U.S. winter storm late this week... ...Pattern Overview and Guidance/Predictability Assessment... The main forecast challenge is the evolution of the slow moving, meandering polar vortex over the Canadian prairies and its southward/southeastward progression (or lack of) by later in the period. Sensible weather impacts include northward streaming Gulf moisture and cyclogenesis along the East Coast and potential wintry weather. The latest deterministic and ensemble guidance now favors keeping the core of the polar vortex north of the US border through day 5 then with the approach of mid/upper level energy out of the Pacific toward California, then the flow over the CONUS becomes more zonal and progressive, which lifts the PV northeast north of the Great Lakes. As this happens, low pressure is still forecast to develop along the stationary/wavy boundary along the Gulf Coast/Southeast U.S. and then lift up the East Coast. There remains run to run variability in how progressive or dominant the West Coast shortwave will be and its impact downstream with the PV, especially by day 6/7. So overall forecast confidence is average to below average by late in the period. The WPC medium-range forecast package was derived initially from the 06Z GFS/00z CMC/00Z ECMWF/00Z UKMET followed by mostly a blend of the 06Z GFS, 00Z NAEFS and 00Z ECMWF/ECENS given some of the model uncertainties later in the period. ...Weather/Threats Highlights... A stationary front across southern Florida will lift northward as a warm front and likely provide a period of enhanced rainfall from the eastern Gulf Coast region to the southeastern U.S. coast midweek. A quick moving shortwave impulse tracking from the lower Ohio Valley into the central Appalachians could bring some meaningful accumulations of snow to portions of the central Appalachians from eastern KY to WV and southwest to southern PA. By midweek, Gulf moisture is forecast to return and expand in coverage across the southern tier states and then up the East Coast in an overrunning pattern north of a stationary front across the northern Gulf. With the cold air in place farther inland, a swath of snow and ice remains possible from portions of the Ohio Valley to the Northeast. The latest Winter Weather Outlook probabilities call for a 30-70 percent chances of snow and ice from upper Ohio Valley to southern New England from Wednesday to Thursday. Focus of upslope snow over the Colorado Rockies near the arctic front will gradually shift to the West Coast as the next Pacific system pushes moisture onshore by Thursday. The Cascades and the Sierra Nevada can expect snowfall to resume and continue into the weekend while periods of rain affect a good portion of California. Temperature-wise, the arctic air mass will sustain a large area of single digit and sub-zero temperatures across the northern to central Plains where readings will be 30-40F below normal from midweek into the weekend. Temperatures reaching 20-35F below normal may extend into the southern Plains and much of the Mississippi Valley by later in the week but uncertainty in the eastern U.S. pattern lowers confidence in exactly how cold the East will be at that time. Most locations west of the Rockies will see a general cooling trend as the arctic air filters in from the northeast. In contrast, temperatures in the Southwest should average slightly above normal. Kong/Taylor 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