Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 PM EST Tue Jan 18 2022 Valid 12Z Fri Jan 21 2022 - 12Z Tue Jan 25 2022 ...Heavy Snow/Ice Threat for the Southeast/Mid-Atlantic/Northeast into Friday/Saturday... ...Overview... Arctic high pressure will push a cold front through the Deep South and East later this week, leading to much below normal temperatures. Moist air fed into the southern and eastern periphery of the cold airmass is increasingly likely to fuel a heavy snow/ice threat from the Southeast to the Mid-Atlantic into Fri. Potential coastal storm development would enhance Mid-Atlanic to Northeast heavy snow potential Sat. Meanwhile, upper ridging builds off/along the West Coast as digging Intermoutain West/Rockies shortwave energy spills snows southward in route to producing a Southwest closed low by the weekend slated to eject across the U.S. southern tier early next week. ...Guidance/Predictability Assessment and Weather Highlights/Threats... Guidance remains in decent agreement with the large scale mean flow over the lower 48 highlighted by an upper ridge off/along the West Coast and upper troughing over the central and eastern U.S.. However, the models have recently been horrendous with run to run continuity with embedded shortwaves that dig through the mean upper trough position. In particular, forecast spread remains a significant timing and amplitude issue with a main shortwave over the east-central to eastern states Fri/Sat that produces major placement differences with a potentially impactful coastal low genesis in the western Atlantic. The 12z ECMWF is faster and more northerly than last nights 00z run. This is consistent with its current trend. The 12z GFS is considerably more progressive than the 12z ECMWF/UKMET/CMC suite, but consistent with its 00z predecessor with its solution placing the shortwave axis farther north than the other deterministics. There is potential for heavy snow, sleet, and freezing rain particularly from the southern Appalachians and Carolinas/Mid-Atlantic into Fri, potentially into the Northeast by Sat. The pattern will also be favorable for lake effect snow given the arctic airmass in place. Into early next week, guidance agrees to hold an amplified mean upper trough over the cooled east-central U.S., ableit with variations with embedded shortwaves set to dig and reinforced the feature underneath a deep low centered just east of the Hudson Bay. Meanwhile out West, models genrally agree to dig shortwave energy southward across the Intermountain West Friday leading to a Southwest U.S/northern Mexico closed low by the weekend. Lead flow response in advance of expected system ejection out across the U.S. southern tier early next week would support a modest swath of overrunning precipitation within a cooled airmass from Texas eastward across the Gulf Coast. The best chance for snow will Fri/Sat down through the Rockies as the shortwave impulse moves through. Other than that, conditions should remain mostly dry, with slightly warmer than normal conditions for the West Coast states underneath the ridge, and above normal temperatures are then expected for the High Plains early next week. Kebede/Schichtel 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 Hazards: - Heavy snow across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Fri-Sat, Jan 21-Jan 22. - Freezing rain across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Fri-Sat, Jan 21-Jan 22. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, and the Southern Plains, Fri-Sun, Jan 21-Jan 23. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Fri, Jan 21. - Much below normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sun-Tue, Jan 23-Jan 25. - Much below normal temperatures across portions of the Northeast, the Central Appalachians, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Fri-Sat, Jan 21-Jan 22. 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