Extended Forecast Discussion NWS Weather Prediction Center College Park MD 146 AM EST Thu Dec 30 2021 Valid 12Z Sun Jan 02 2022 - 12Z Thu Jan 06 2022 ...New Year's weekend heavy rain/thunderstorm threat for the South/Southeast/Mid-Atlantic as a heavy snow threat shifts from the Midwest/Great Lakes/Northeast down through the Appalachians... ...Coastal heavy rain and mountain focusing heavy snow threat Sunday-Wednesday from the Northwest to north-central California... ...Guidance and Predictability Assessment... The WPC medium range product suite was based off a compromise of the 12Z ECWMF/CMC/EC ensemble and GEFS means with inclusion of the NBM. The CMC and ECWMF have been consistent with the overall pattern clustering and evolution the past few days while the GFS continues to struggle. The inclusion of the means help to reduce timing and phasing differences for the multiple features during the extended period. This model blend helps to maintain continuity. ...Weather/Hazards Highlights... Coastal rains and mountain snow is expected across parts of the Northwest and California as an amplified upper trough moves through the West, then crosses the Rockies spreading snow before entering the Plains. Once in the Plains the ejected energies will help spawn systems to enhance an emerging central to eastern U.S. precipitation focus. There will be a wavy frontal system across the central U.S. with deep moisture streaming over it, providing a focus for heavy rainfall from the Mid-South eastward to the Mid-Atlantic and Northeast. There is a potential for localized runoff issues from the Mid-South and Ohio Valley/Northeast down into the Mid-Atlantic and South/Southeast this weekend. Expect widespread record breaking warmth this weekend in advance of the starkly temperature contrasting front over the South and East. Cold Canadian air surging in behind the front across the central and eastern states will spread a heavy snow/ice threat from the Midwest/Great Lakes/Northeast down through the southern Appalachians. Guidance remains quite varied with details, but shows some signal that frontal wave genesis could translate to an organized coastal low over the western Atlantic Sunday into Monday that could offer some threat to coastal areas and marine interests. Another multi-day round of precipitation will move through the Pacific Northwest and northern/central California Sunday through midweek. Strong gust winds will also accompany the heavy precipitation. During this time the upper level pattern slowly transitions as part of a developing Omega block over the Northeast Pacific in the form of a deeply amplified closed upper low/trough digs slowly southward from the Gulf of Alaska to offshore the Northwest. Heavy coastal rains and heavy mountain snows will focus from the Cascades to the Sierra inland to the north-central Great Basin/Rockies. Campbell/Schichtel 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 Great Basin, the Pacific Northwest, and the Southwest, Sun-Tue, Jan 2-Jan 4. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, the Ohio Valley, and the Tennessee Valley, Sat-Sat, Jan 1-Jan 2. - Heavy snow across portions of the Great Basin, the Northern/Central Rockies, and the Pacific Northwest, Mon-Tue, Jan 3-Jan 4. - Heavy snow across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sat, Jan 1. - Severe weather across portions of the Middle/Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Sat, Jan 1. - High winds across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sat, Jan 1. - Much below normal temperatures across portions of the Plains, the Rockies, the Great Basin, and the Upper/Middle Mississippi Valley, Sat, Jan 1. 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