Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Mon Dec 19 2022 Valid 12Z Thu Dec 22 2022 - 12Z Mon Dec 26 2022 ***Bitterly cold temperatures expected over all of the Plains and Midwest states, and very cold weather reaches the Gulf Coast region*** ***Significant winter storm over the Eastern U.S. late in the week will likely cause travel disruptions*** ...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. An upper ridge axis near the West Coast 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 with widespread rain, snow, and strong winds, and this will disrupt those that are traveling late this week. The intensifying low then lifts northward across eastern Canada by Saturday with widespread gusty winds and lake effect snow in its wake. A southern stream shortwave will likely cross the Gulf Coast region around Sunday, but most of the precipitation from this is currently expected to remain offshore. The arctic airmass is expected to gradually moderate going into the weekend. ...Guidance/Predictability Assessment... The 00Z model guidance suite has now come into better agreement on the expected synoptic scale set-up for the major Eastern U.S. low pressure system, and recent model trends have stabilized after significant model differences over the past couple of days. The GFS is still a little slower with the low track and slightly farther south with the main closed low over the Ohio Valley, whilst the UKMET is slightly faster than the consensus with the low lifting north from upstate New York to Quebec. The 00Z ECMWF/CMC serve as a good middle ground solution and these two models were weighted most in the WPC fronts/pressures forecast for Thursday and Friday, along with some of the GFS and the ensemble means. There has also been a trend for a better defined southern stream shortwave that the GFS has portrayed over the past few runs, and now the other deterministic guidance is showing some support for this as the trough tracks from the southwestern U.S. to the Gulf Coast region, with the GFS remaining most amplified with this feature. Towards the end of the forecast period, more significant model differences are apparent across the Northeast Pacific with the next storm system to affect the Pacific Northwest, so more of a ensemble mean based approach works well for Sunday and into Monday. ...Weather/Hazard Highlights... One of the main things that will make weather headlines next week will continue to be the massive expanse of frigid temperatures across basically all of the central and eastern U.S. east of the Rockies through the end of the week, with hard freezes likely for much of the Deep South and well into Texas. 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. Cold weather will also reach well into Florida in time for Christmas weekend, with lows falling into the 30s and 40s. The other weather story that will be quite impactful will be expected development of an intense low pressure system that develops over the Ohio Valley and rapidly intensifies as it lifts northward across the Great Lakes and then eastern Ontario/Quebec by early Saturday. The expected inland track of the main low will result in periods of moderate to heavy rain from the Mid-Atlantic to New England 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. There is even the potential for some minor flooding across portions of central and eastern New England as 2-3 inches of rainfall is becoming more likely with strong moisture advection from the Atlantic ahead of the main cold front. Therefore, a Slight Risk of excessive rainfall is now in effect for the Day 5 period on Friday to account for this. 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, especially on Sunday and into Monday. 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