Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM EST Wed Feb 10 2021 Valid 12Z Sat Feb 13 2021 - 12Z Wed Feb 17 2021 ...Arctic air remains entrenched across the Plains and MS Valley but should gradually moderate by early to middle of next week... ...Back-to-back winter weather events possible for the northeastern U.S. followed by the south-central U.S to the Northeast... ...Unsettled weather returns to the Pacific Northwest and the interior western U.S.... Models and ensemble are in general agreement that a large dome of arctic air will remain entrenched across the northern and central U.S. into the weekend as the center of a meandering polar vortex begins to slide eastward across the Great Lakes. Despite this general agreement, models have continued to make further adjustments to important shortwave energies that rotate around the vortex. The GFS has backed off on the amounts of upper-level vorticity ejecting across the central Plains, resulting in less robust frontal wave development over the Great Lakes later in the weekend. The ECMWF on the other hand, has slowed down the eastward progression of the polar vortex, resulting in a better-defined surface inverted trough across the Great Lakes. Thus, both models have come into better agreement with each other as a winter weather event appears to be in the offing for the northeastern U.S. through the weekend. This is in response to frontal wave developments across the Southeast where a decent rain event can be expected near the coast. Meanwhile, a couple of low pressure systems from the Pacific are forecast to move steadily from northwest to southeast across the western U.S. during the medium-range period. They will likely bring a couple of rounds of mountain snows and valley rains from the Pacific Northwest toward the Great Basin and the central to southern Rockies. The first 500 mb wave is forecast to weaken in a confluent flow as it ejects onto the Plains. The second wave crossing the Southwest on Sunday will then move east across the southern Plains on Monday and into the MS Valley on Tuesday, and will likely lead to low pressure development in the Gulf of Mexico. There is considerable run-to-run model variability regarding this system. Nevertheless, model consensus has been trending toward a faster northeastward motion of this system across the Southeast early next week. There continues to be good model signals for a large swath of snow and/or ice to extend from the south-central U.S. to the Northeast early next week as the arctic high pressure system retreats into eastern Canada. With the arctic high pressure system remains in firm control, many cold temperature records will be likely down the Plains and into the lower to mid MS Valley this weekend to early next week. Record lows/low maxima should be most numerous on Sunday before the airmass gradually moderates early next week. Models continue to signal periods of snow through the weekend across the ranges of WA/OR, southern ID, WY, northern CA, northern NV and UT. On Sunday, the southwest trough is forecast to produce snows in the ranges of AZ/UT/NM/CO. By early next week, snows should wind down in the southern Rockies but will likely redevelop in the mountains of western WA/OR as the next system approaches from the northeast Pacific Ocean. 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