Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EST Tue Dec 28 2021 Valid 12Z Fri Dec 31 2021 - 12Z Tue Jan 04 2022 ...Bitterly cold temperatures to continue Thursday-Saturday across portions of the Northern Plains... ...Heavy rain across across parts of the Tennessee/Ohio Valley New Year's Eve/New Year's Day... ...Overview and Guidance... An amplified ridge will be anchored over the northern Pacific Ocean through this period, which will continue to support an upper trough over the western U.S.; this pattern will favor very brisk temperatures. A separate piece of energy within the larger scale trough should track a little offshore southern California Thursday-Friday. The overall pattern will begin to transition by the weekend as a new ridge builds farther west, leading to a deepening trough over the Northeast Pacific and renewed moisture feed into the Northwest U.S. The trough and associated moisture will track into the central and eastern part of the country. A much weaker wavy frontal system will depart from the East late this week. The latest model guidance continues to have similar patterns and evolution through the extended therefore choose to take a multi-model approach; this also helps maintain continuity from the previous forecast. The WPC blend used the 12Z ECWMF/CMC/UKMET, 12/18Z GFS initially and gradually increased the use of the EC ensemble means and the GEFS means trough the middle and later periods. ...Weather/Hazards Highlights... Lingering showers are expected across parts of the Southeast near a wavy cold front along with snow over the Northeast near the parent low pressure. Scattered to widespread showers and thunderstorms will move across the Plains towards the eastern U.S. as a low pressure system and associated fronts develops. A plume of deep moisture will accompany this system and intensify over the Tennessee Valley; where conditions will be very favorable for moderate to heavy rainfall to occur. There will be an elevated risk for local flooding concerns as this region has had recent soaking rains to lower local flash flood guidance. Expect some wintry weather in the northern side of the precipitation shield--from parts of the Northern/Central Plains through the Midwest, Great Lakes, and Northeast. The intensity and storm track, along with any waves along the boundary, will determine the the location and amounts. Heavy mountain snow will spread across the Pacific Northwest and Northern Rockies Thursday ahead of an approaching shortwave. Abundant moisture will be directed onshore southern California with the upper trough/possible closed low positioned offshore. Moderate to locally heavy rainfall may be possible in the lower elevations; with snow spreading across the higher terrain especially over the Colorado Rockies with the merging of northern and southern stream upper systems. A third system nearing the Pacific Northwest will usher in an additional round of heavy mountain snow by the weekend. Lower elevation rain could become moderate to heavy from the Pacific Northwest into northwestern California by early next week. The frigid airmass over the northern tier should persist through at least Saturday with the daily readings averaging 20 to 35 degrees below average for the end of the year. A few locations in far northern North Dakota and Montana may stay below 0 F even into next weekend, despite temperatures moderating slightly as the week progresses. Gusty winds will create some dangerous wind chill values of below 0 F. Temperatures are also likely to be below normal by about 5-15 degrees across the rest of the western U.S. through Sunday. Warm and mild weather is expected across the Southeast and Mid-Atlantic into Sunday, with daily temperature readings of 15 to 25 degrees warmer than normal from the Southern Plains to the Ohio Valley and east into the Mid-Atlantic region. Campbell/Rausch 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 Hazards: - Heavy precipitation across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Sun-Mon, Jan 2-Jan 3. - Heavy precipitation across portions of the Southwest, Thu-Fri, Dec 30-Dec 31. - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Sat-Sun, Jan 1-Jan 2. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Thu, Dec 30. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Sat, Jan 1. - Severe weather across portions of the Southeast, the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Sat, Jan 1. - High winds across portions of the Central Rockies, the Central Plains, the Northern Plains, and the Northern Rockies, Thu, Dec 30. - Much below normal temperatures across portions of the Upper Mississippi Valley, the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Thu-Sat, Dec 30-Jan 1. - Much below normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sat-Sun, Jan 1-Jan 2. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml