Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM 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, with anormally cold weather reaching the Gulf Coast region as well... ...Significant winter storm from the Midwest to the Northeast 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 bring arctic air and 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. Temperatures from the central to eastern U.S. should gradually moderate from west to east this weekend and beyond. ...Guidance/Predictability Assessment... The latest guidance continues to show much better agreement on the expected synoptic scale set-up for the major Eastern U.S. low pressure system. The 00z and 06z runs of the GFS continued to show a slower and slightly farther south track with the main upper low compared to the ECMWF, CMC, and UKMET, but the new 12z GFS run (available after forecaster generation time) came much more in line with the consensus. There also remain some track/placement differences on the upper and surface lows as they track northward into Canada. Some timing/amplitude differences, but the guidance also seems to have a better handle on a shortwave tracking from the Southwest to the Gulf Coast as well this weekend. For both, a general model compromise seemed to work well as a starting point for the updated WPC forecast. Another shortwave riding the top of the Western U.S. ridge showed plenty of uncertainty around Sunday-Monday regarding amplification over the central U.S.. Again, the main outlier here was a much weaker 06z GFS, but the 12z looks better. A blend of the ECMWF, CMC, with the ensemble means worked well for this system. Overall, needed changes to the previous WPC forecast were minimal. ...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 single digit temperatures for overnight lows through the weekend. 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 the 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 to the west from the Midwest to the northern Ohio Valley and Great Lakes, and the central/northern Appalachians. 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, especially if this rainfall falls over regions that may still have a snow pack from last weeks storm. 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. Santorelli/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