Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EST Mon Jan 17 2022 Valid 12Z Thu Jan 20 2022 - 12Z Mon Jan 24 2022 ...Snow/ice are possible along the Eastern Seaboard late this week and weekend on the eastern periphery of an Arctic high... ...Overview... Arctic high pressure will push a cold front across the East late this week, leading to very cold temperatures across the central and eastern CONUS. Moist air spilling into the southern and eastern periphery of the chilly airmass could produce wintry weather with light amounts over Texas Thursday to currently better chances for notable amounts over the Carolinas and into the Mid-Atlantic late this week, possibly spreading into the Northeast this weekend, though uncertainty remains high with the details given model guidance disagreements. Meanwhile, upper ridging is expected for the West, with the exception of a shortwave digging south that may produce a closed low over the Southwest this weekend. ...Guidance/Predictability Assessment... The 00Z/06Z suite of model guidance begins the period Thursday with fairly good agreement for the overall pattern--upper troughing across the central and eastern U.S. with a strong Arctic high at the surface, and a high over the eastern Pacific with a ridge axis into the West. A couple of shortwaves that are significant to the pattern then have somewhat low predictability Friday and beyond. In the East, a shortwave moving through the persistent eastern trough shows considerable differences in placement and strength by late Friday into Saturday through the Ohio Valley to Eastern Seaboard, which produces placement differences with a potentially impactful surface low in the western Atlantic. The 00Z and incoming 12Z ECMWF are generally on the western side of the model spread with these features, leading to more precipitation over the Eastern Seaboard, while the GFS runs have been suppressed farther east into the Atlantic. The 00Z cluster analysis and ensemble member low plots showed that the GEFS and EC ensembles followed their operational runs, with GEFS members farther east than EC ensemble members. Tended to favor a position for the surface low near the 00Z EC ensemble mean as somewhat of a compromise, which was not too far off from the 00Z CMC position, but the 12Z CMC is now showing a more suppressed pattern. This forecast approach led to a trend west of the previous WPC forecast with the surface low and frontal placement, but not nearly to the western extent of the ECMWF runs. Meanwhile in the West, models all tend to show shortwave energy coming into British Columbia Thursday with at least a portion of the energy diving south across the Intermountain West Friday, and could close off a southern stream low in the Southwest by around Saturday. There has been good model agreement for a couple cycles now for the low to close off at some point this weekend (recent 12Z guidance is showing later compared to the 00Z/06Z guidance), so felt it was prudent to include a closed low for the forecast update, but there is inherent uncertainty as the shortwave energy is stemming from the data-poor Pacific Ocean. Early next week, guidance all shows troughing remaining strong across the East given a deep low centered just east of the Hudson Bay, but with some variations in shortwaves around it. The WPC forecast was based on a blend of deterministic guidance early on, including and increasing weighting of the GEFS and especially the EC ensemble guidance as the period progressed. ...Weather Highlights/Threats... A massive Arctic high will spread from the central to eastern U.S. Thursday through Saturday behind a potent cold front, producing much below normal temperatures. Lows of 15-25F below average are likely across the north-central U.S. Thursday and Friday morning and across the interior Northeast Friday and Saturday. Lows in the teens are likely as far south as Texas, while in parts of the Midwest temperatures could stay below 0F even for highs on Thursday. High temperatures should be 20-30F below normal for the Plains to the Eastern Seaboard late this week before moderating early next week. On the southern and eastern periphery of this high, moist air spilling into the cold air could lead to wintry weather on the northern periphery of precipitation spreading across the Gulf Coast states and then possibly up the Eastern Seaboard. Once again, uncertainty with the evolution of shortwaves and surface lows leads to low confidence in placement of wintry precipitation and precipitation type in some locations at this time, but the current forecast shows the potential for notable snow, sleet, and freezing rain particularly in the Carolinas and southern portions of the Mid-Atlantic for the latter part of the week, with some chance of spreading into the Northeast this weekend. The pattern will also be favorable for lake effect snow given cold air in place and generally westerly winds. In the West, the best chance for precipitation will be Thursday and Friday with mountain snow in 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. Tate 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