Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 PM 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... Unsettled and active pattern evolving over the CONUS during the period with multiple systems affecting both the west coast to the central/east coast. The large scale pattern focuses on the evolution and eventual shearing of the polar vortex over the Great Lakes to New England with an eventual Pacific shortwave that digs a trough over the western U.S. by late in the period. This setup favors a continuation of the much below normal temperatures and Arctic air over the Northern Plains and Upper Midwest that slowly sags southward across more of the CONUS by day 7. Ahead of that, interactions along a wavy/stationary boundary in the Gulf to Southeast will lead to several possible low pressure systems riding up the coast (or offshore) and this brings some potential sensible weather impacts including wintry precipitation. For the West Coast, after a brief reprieve in active weather, the approach of a stronger storm system will bring enhanced moisture with potential for heavy snow for the higher elevations and rainfall to the lower valleys. The guidance over the last several days has cycled and struggled with the southward push of the polar vortex. Recent trends show that there will be less of a southward intrusion as the PV gets sheared out to the east portions of the Great Lakes to northern New England as the Pacific energy quickly moves toward the West Coast and Rockies. The models are in reasonable agreement with this idea, though the deterministic 00Z CMC was less favored as it was considerably slower while the other models showed fair amount of agreement. For the west coast shortwave, the typical fast bias GFS was noted while the ECMWF was slower. Overall, while large scale/upper level pattern was fairly predictable, the individual shortwaves and low pressure tracks, especially for the eastern U.S. had lower than average forecast confidence, especially day 5 onward. As such, the WPC blend was derived initially from a near equal blend of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z UKMET followed by mostly a ECMWF/ECENS and GFS/GEFS blend for day 5 to day 7. ...Weather/Threats Highlights... A swath of winter precipitation remains in the forecast for portions of the Ohio Valley to the Mid-Atlantic early in the period while ice accumulations could be significant for portions of KY at the start of day 3. As an arctic front on the heels of a very strong Canadian high pressure (high near 1046 mb forecast), upslope/terrain snow over the Rockies could be heavy while another system could organize and lift up through the Mid-Atlantic this weekend bringing another round of winter precipitation to portions of the Appalachians and Mid-Atlantic to Northeast. There is considerably uncertainty and large variability for the weekend forecast for the East Coast however. Temperature wise, the Arctic 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. 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