Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Dec 20 2022 Valid 12Z Fri Dec 23 2022 - 12Z Tue Dec 27 2022 ***A major winter storm from the Midwest to the Northeast late in the week will likely cause blizzard conditions and travel disruptions, along with bitterly cold temperatures and strong winds to close out the week*** ...Synoptic Overview... An impressive dip in the jet stream and a highly anomalous upper trough/closed low over the Ohio Valley on Friday will sustain explosive surface cyclogenesis across the Great Lakes with central pressures falling well below 980 mb by Friday evening. This will cause an incredibly strong cold front to sweep across the Eastern U.S. with frigid temperatures from the Plains to the East Coast in time for Christmas weekend. The low lifts northward across Quebec on Saturday with lake effect snow continuing, and heavy rainfall exits New England. The huge arctic surface high then settles across the south-central U.S. and then modifies with gradually warming temperatures going into Christmas Day and early next week. Farther west, multiple shortwave impulses will pass over the Pacific Northwest and result in periods of heavy rain and high mountain snow from western Oregon to western Washington. ...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 with a slight eastward adjustment in the placement of the low (12Z ECENS slightly northwest of 00z model consensus). Therefore, a general model compromise suffices as a starting point in the forecast process, even though some smaller mesoscale differences in the guidance remain. There also remains a good model signal for a better defined southern stream shortwave, with the CMC remaining most amplified with this feature as it drops southward over eastern Mexico. Towards the end of the forecast period, model depiction has improved with an amplifying trough across the Northeast Pacific early next week with the next storm system to affect the Pacific Northwest. However, there is more model spread with a trough building over the Midwest by next Tuesday, so more of the ensemble means were incorporated for days 6 and 7. ...Weather/Hazard Highlights... The main weather story is the 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 with blizzard conditions from the Midwest to 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, especially if this rainfall falls over regions that may still have a snow pack from last week's storm. Therefore, a Slight Risk of excessive rainfall remains in effect for the Day 4 period on Friday to account for this. Strong and potentially damaging winds are likely across portions of Ohio, Pennsylvania, and western New York Friday and into early Saturday as a sting jet mixes strong winds aloft to the surface. The truly massive expanse of frigid temperatures across basically all of the central and eastern U.S. east of the Rockies will also continue to make weather headlines to close out the week, with hard freezes likely for much of the Deep South and well into Texas. There will be widespread subzero readings for both overnight lows and even daytime highs from the Northern Rockies to the central/northern Plains and the Upper Midwest, especially on Friday and Saturday. Although the frigid airmass will modify some as it reaches the Ohio/Tennessee Valley region, it will still be impressively cold with widespread 0s and 10s for high temperatures, and some subzero overnight lows. Hard freeze conditions are expected for much of the Deep South and Texas Thursday night through at least Friday night, with lows well into the 10s 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 across much of the Peninsula. 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