Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 PM 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. Expect the low to lift northward across western Quebec on Saturday with lake effect snow continuing, and heavy rainfall exiting New England very early in the day. The trailing arctic surface high will settle across the south-central U.S. and then modify 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 over western Oregon/Washington, with some snow extending into the northern Rockies. Moderately above normal temperatures should gradually spread over more of the West and then extend into the High Plains. ...Guidance/Predictability Assessment... Latest guidance offers good agreement on the large scale upper pattern that features a West Coast ridge and eastern trough, with Pacific shortwaves rounding the ridge and feeding into the mean trough. Consensus shows the mean ridge beginning to broaden as it pushes inland by day 7 next Tuesday while troughing approaches the West Coast. Detail differences for the initial Ohio Valley/Great Lakes storm and associated frontal system as well as northeastern Pacific features that brush the Northwest through the weekend are generally within the typical spread and guidance error for the time frame involved, so an operational model composite worked well for depicting significant features in the updated forecast. By days 6-7 Monday-Tuesday there are more noticeable detail differences that recommend a trend toward a model/ensemble mean blend. These include specifics within the eastern Pacific mean trough, and within the eastern North America mean trough one shortwave dropping into the central/eastern U.S. and possibly a separate one dropping from Canada into the Great Lakes region. Recent ECMWF runs have been more pronounced with this northern feature than most other models but the new 12Z CMC has trended much closer to the 00Z ECMWF, while the 12Z ECMWF has backed off with this feature's strength somewhat. An intermediate solution seems reasonable at this point given the split in the guidance. ...Weather/Hazard Highlights... The main weather story is the expected development of an intense low pressure system over the Ohio Valley into the Great Lakes, with the surface low then tracking into western Quebec by early Saturday. The expected inland track of the main low will result in periods of moderate to heavy rain spreading from the Mid-Atlantic to New England late Thursday into late 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 overnight lows and even some 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. These readings will equate to anomalies of 20-35F or so below normal over a broad area, and some daily records for coldest morning lows/daytime highs appear likely even with the challenging comparisons to events from 1989 and 1983 (among other years depending on location). Elsewhere, the Pacific Northwest should see multiple episodes of rain and higher elevation snow as persistent onshore flow advects moisture inland while shortwaves track around the northern side of the mean ridge aloft near the West Coast. Some moisture should extend into the northern Rockies where moderate to locally heavy snow may fall. By late in the weekend the southern part of the precipitation area should trend lighter as the upper ridge briefly rebounds. Then during the first part of next week expect the rain and mountain snow to become heavier and push farther southward, eventually reaching into California, as a larger scale upper trough approaches and the ridge axis pushes farther inland. Dry weather should continue across the southwestern U.S. under the upper ridge. Rausch/Hamrick Hazards: - Heavy precipitation across portions of the Pacific Northwest, Fri-Sat, Dec 23-Dec 24. - Heavy precipitation across portions of the New England, Fri, Dec 23. - Heavy rain across portions of California and the Pacific Northwest, Tue, Dec 27. - Heavy rain across portions of the New England, Fri, Dec 23. - Heavy snow across portions of the Great Lakes and Northeast, Fri, Dec 23. - Flooding possible across portions of the Northeast. - High winds across portions of the Midwest, Ohio Valley, Appalachians, and Northeast, Fri, Dec 23. - Much below normal temperatures across portions of the Pacific Northwest, Northern Rockies, and High Plains, Fri-Sat, Dec 23-Dec 24. - Much below normal temperatures across portions of the Plains, Midwest, Southeast, Mid-Atlantic, and Northeast, Fri-Sun, Dec 23-Dec 25. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Tue, Dec 25-Dec 27. - Heavy snow across portions of the Alaska Panhandle, Fri-Mon, Dec 23-Dec 26. - Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Dec 23-Dec 26. 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