Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1256 PM EST Wed Feb 10 2021 Valid 12Z Sat Feb 13 2021 - 12Z Wed Feb 17 2021 ...Arctic air forecast to set multiple cold temperature records this weekend across the Plains and MS Valley but should gradually moderate by early to middle of next week... ...Stormy pattern with multiple winter precipitation events... Models and ensembles are in general agreement that a large dome of arctic air will surge south across central U.S. into the weekend as the center of a polar vortex slides eastward across the Great Lakes. Models have continued to make further adjustments to important shortwave energies that rotate around the vortex. Both the 06z GFS and 12z Tue/00z Wed ECMWF models have come into better agreement with each other as shortwave energy emerges from the southern Rockies onto the southern Plains on Sunday, resulting in an area of likely snow in northwest Texas across central to western OK and KS. As this system ejects to the northeast on Mon 15 Feb., the area of heavy snow northwest of low extends across mid MS Valley to the Great Lakes. Further southeast, significant mixed sleet and freezing rain area expected from the lower MS Valley to the TN Valley, central Appalachians, and Piedmont of NC/VA/Mid Atlantic. On Tue 16 Feb, the parent low moves across the Great Lakes into Canada, with cold advection producing lake effect snow in MI and northwest PA/western NY. The low weakens, but not before producing a round of snow in northern NY/northern New England with potential for a change over to sleet/freezing rain/rain in southern New England/southeast New York before the system departs. Widespread moderate rain is expected further south in the southeast to the coastal mid Atlantic states. Confluent flow in the west results in a series of progressive 700 mb waves that produce rounds of snow in the terrain, starting on Day 4 Sat/early Sun from the WA/OR Cascades into the ranges of northern CA and northern Great Basin. On Day 5, the first wave crosses NM/southern CO, bringing snow to the mountains there before the wave exits on the Plains. The next Pacific northwest disturbances brings snow back to the WA/OR Cascades. On Monday the water vapor transport onshore and inland combines with a wave to produce snow starting with the WA/OR Cascades and continuing to the ranges of ID to western WY and northern UT. On Tuesday 16 Feb, the mid-upper level northwest flow brings moisture and light-moderate snows from the northern to central Rockies and Great Basin. With the arctic high pressure system in the Plains, 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 next week. The WPC medium-range forecast package was derived from a blend of the 12Z-00z ECMWF/00z EC Ensemble mean, and the 06Z GFS/GEFS Mean. The large scale pattern looks similar across most models/ensembles with a reamplifying 500 mb ridge off the west coast and persistent central US upper trough, and ridge off the southeast coast. The 00z Canadian deamplified the ridge off the west coast and became a progressive outlier with systems across the country, so was not included in the blend. Petersen/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