Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 AM 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 00 UTC ECMWF/UKMET trended deeper and less progressive with the inland shortwave and subsequent coastal system. The 00 UTC GEFS also trended in that direction. The 00 UTC GFS was slightly less progressive and northward shifted. The 00 UTC Canadian seems in line with its 12 UTC run. It does seem noteable that the 00 UTC model trends now seem more favorable for significant system develpment. However, it has been an annoying recent trend for 00 UTC guidance to trend slower vs 12 UTC guidance, possibly due to diurnal initialization data sensitivity. Hopefully, model solutions will continue to converge on a more common solution. Accordingly and in anticipation, the earlier released WPC product suite was primarily derived from the most amplified ensemble mean solution, the ECMWF ensemble mean. Manual adjustments were further applied to increase shortwave amplitude smoothed out by the mean and to maintain max WPC product continuity. 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 cold air 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. 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 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