Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Dec 18 2022 Valid 12Z Wed Dec 21 2022 - 12Z Sun Dec 25 2022 ***Bitterly cold temperatures likely to settle over the northern Plains, spreading south and east through the end of the week*** ***Potential for a significant winter storm over the Eastern U.S. late in the week*** ...Synoptic Overview... An amplifying upper level pattern across the continental U.S. will result in a very impactful week of severe cold and winter weather across the eastern two-thirds of the nation. A building ridge axis near the West and a potent blocking high over Greenland and northeast Canada will provide the means for a lobe of the polar vortex to break off and sink southward across the north-central U.S., and this will herald the arrival of the coldest airmass of the season by far for most locations east of the Rockies. This potent upper trough will then spur surface cyclogenesis across the Eastern U.S. by the end of the week, and will likely cause some challenges for travelers. The intensifying low then lifts northward across eastern Canada by Saturday with widespread gusty winds and lake effect snow in its wake. ...Guidance/Predictability Assessment... The 00z guidance continues to come into closer alignment with respect to the synoptic pattern over the lower 48. The last few runs of the GFS/GEFS have especially come into better agreement with the EC/CMC/UKMET guidance suite. Some differences in the orientation of the upper trough exist on days 3 and 4 over the Great Lakes/southern Canada, but seem to resolve themselves after day 5. The 00z EC/CMC signaled a closed upper low over the Ohio Valley on day 6 while the 06z GFS and 00z UKMET kept the trough open. Placement and tilt of trough remained reasonably similar throughout the period. The medium range model blend consisted of the 00z EC/UK/CMC/WPC and 06z GFS through day 4. The 06z GEFS was introduced on day 5 to moderate the strengthening upper low over the Midwest. The 00z CMCE/ECE were introduced on day 6 and continued through day 7. ...Weather/Hazard Highlights... The main weather story that will make weather headlines next week will be the massive expanse of frigid temperatures from the Northern Rockies/Northern Plains to the Midwest through the middle of the week, and then reaching the Gulf Coast and much of the Eastern U.S. by Friday and into the weekend. An extremely strong arctic front will usher in the coldest air of the season by a considerable margin with the expectation of widespread subzero readings for overnight lows from the Northern Rockies to the central/northern Plains and the Upper Midwest, reaching as far south as northern Oklahoma and southern Missouri. There will likely even be subzero high temperatures from central Nebraska/central Iowa and points northward on Thursday and Friday. Although the frigid airmass will modify some as it reaches the Ohio/Tennessee Valley region, it will still be impressively cold with widespread 0s for overnight lows by the end of the week and into early Saturday, and even some subzero readings across portions of Indiana and Illinois. Hard freeze conditions are becoming increasingly likely for much of the Deep South and Texas Thursday night through at least Friday night, with lows well into the teens and low 20s, even to the Gulf Coast. Many of the daily records during this time were set during the historic arctic blast of 1983, so this will tend to limit the extent of new records being set. Attention also turns to the expected development of a strong low pressure system that evolves from the merger of a frontal low over the Ohio Valley/Great Lakes and a coastal low near the Mid-Atlantic and southern New England. The expected inland track of the main low over Upstate New York will tend to keep the majority of the precipitation as rain across the East Coast and the Interstate 95 corridor late Thursday into Friday, and moderate to heavy snow from the central/northern Appalachians and points west across the northern Ohio Valley and Great Lakes. Strong winds will likely be an issue as well owing to the strong pressure gradient that develops. Elsewhere, the Pacific Northwest should see multiple episodes of rain and higher elevation snow as persistent onshore flow advects moisture inland, with the highest totals over the Washington Cascades and the Olympics. Some of this moisture will likely make it to the Northern Rockies in the form of moderate to locally heavy snow, especially near the Idaho/Montana border. Dry weather is expected to continue across the southwestern U.S. with the upper ridge in place. Hamrick 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml