Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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 latest model guidance continues to show a polar vortex meandering over the Canadian prairies through the medium-range period with an arctic air mass engulfing much of the northern and central U.S. Farther south, models are again trending slightly slower on the southward and southeastward expansion of the arctic air into the southern tier states. Meanwhile, models are bringing additional moisture from the Gulf into the Deep South by the midweek, and will likely overrun the cold air already in place across the Northeast under a general southwesterly flow aloft. The consensus of ensemble means indicate at least modest cyclogenesis just off the New England coast late this week. Farther inland, models continue to suggest that upper-level impulses exiting the central Rockies under the polar vortex will bring a swath of mostly light snow across the north-central U.S. before possibly interacting with the system off New England later this week. Under this setup, parts of the Northeast should experience some form of wintry weather later this week although the details are far from certain at this time. Elsewhere, upslope snows will likely linger over the Colorado Rockies in the vicinity of the arctic front. The West will see more moderate temperatures but with a general cooling trend as the arctic air filters in from the north, with a continued slowing trend for the arctic front to move across the West. By late this week, models are now in good agreement that moisture from the next Pacific will mostly likely reach the middle of the West Coast and will persist into the weekend. 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. More of the ensemble means were used on Days 6 and 7 as well given the increasing model uncertainties. ...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 the favored terrain areas of eastern Kentucky and West Virginia. 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 southern tier into the Northeast. The latest Winter Weather Outlook probabilities call for a 30-60 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 spreads into 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. Northern areas should start out near to moderately below normal and trend a bit colder, while the Great Basin/Southwest start the weak moderately above normal and then decline toward normal (or slightly below in parts of the Great Basin). 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