Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EST Wed Dec 29 2021 Valid 12Z Sat Jan 01 2022 - 12Z Wed Jan 05 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 primarily derived from a composite of best clustered guidance from the 06 UTC GFS/GEFS, the 00 UTC ECWMF/ECMWF ensemble mean and the 13 UTC National Blend of Models. The synoptic pattern for this period still shows above average predictability while the blend tends to mitigate lingering timing and phasing differences with embedded systems that increases local focus uncertainty. WPC product continuity is well maintained. ...Weather/Hazards Highlights... An amplified upper trough over the West will eject across a very snowy south-central Rockies to the Plains Saturday. Ejecting energies in two streams will then generate systems to enhance an emerging central to eastern U.S. precipitation focus. Deep moisture streaming into a strong and wavy frontal system offers potential for heavy rainfall and localized runoff issues from the Mid-South and Ohio Valley/Northeast down into the Mid-Atlantic and South/Southeast this weekend. Some storms across the South may also be severe, per SPC. Expect widespread record breaking warmth this weekend in advance of the starkly temperature contrasting front over the South and East. Cold post-frontal air working across the central to eastern U.S. should meanwhile 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. Well upstream, Sunday-Wednesday will become quite stormy for the Northwest and the north-central California as 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. This will result in a multi-day heavy wind/precipitation event. Heavy coastal rains and heavy mountain snows will focus from the Cascades to the Sierra inland to the north-central Great Basin/Rockies. 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