Extended Forecast Discussion NWS Weather Prediction Center College Park MD 218 PM EST Tue Dec 28 2021 Valid 12Z Fri Dec 31 2021 - 12Z Tue Jan 04 2022 ...Bitterly Cold Temperatures to continue into New Year's Day across the Northern Rockies/Plains... ...Heavy New Year's Eve to New Year's Day Heavy Snows Threat for the south-central Great Basin/Rockies... ...Heavy Rain/Convection threat from the Mid-South/Ohio Valley to the Mid-Atlantic/Southeast New Year's Eve into New Year's Day... ...Backside Heavy Snow threat meanwhile shifts from the Central Plains and Midwest/Great Lakes/Northeast down through the southern Appalachians... ...Multi-day Heavy rain and terrain snow threat Sunday-Tuesday from the Northwest to North-Central California... ...Guidance and Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of the 06 UTC GEFS ensemble mean and 00 UTC ECMWF/Canadian/ECMWF ensemble mean that offer a similar synoptic pattern evolution days 3-7 in a pattern with better than normal forecast spread and predictability. The composite blend tends to mitigate small-moderate scales timing and phasing differences with embedded systems that remains an issue that increases local focus uncertainty. The latest few runs of the GFS have shown less stellar run to run continuity. ...Weather/Hazards Highlights... An amplified ridge built over the east-central Pacific will allow the digging of a series of potent shortwave troughs and frontal systems into the West Friday into Saturday along with cold temperatures and unsettled weather conditions highlighted by south-central Great Basin/Rockies heavy snows as locally enhanced by favored terrain. Ejecting upper trough energies in two streams out from the West will generate systems and enhance an emerging east-central U.S. precipitation focus with progression out across the central and eastern U.S. as an upper ridge erodes over the Southeast. Heavy rains and some local runoff issues may develop and spread northeastward across the Mid-South and OH Valley/Northeast to ring in the new year. SPC also offers some risk of severe weather over the South Friday into Saturday. Arctic air on the backside will meanwhile spill through the n-central U.S. to also spread a risk of heavy snow from the central Plains to the Midwest/Great Lakes. Wavy frontal progression should shift organized rains more into the Mid-Atlantic/Southeast into Sunday before working offshore. Post-frontal cold air intrusion may then favor lingering Sunday snow/ice chances from the southern Appalachians and vicinity up through the Northeast. Sunday-Tuesday of next week then looks quite active for the Northwest and north-central California 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. Expect an increasingly windy multi-day heavy precipitation episode with moderate to heavy coastal rains and heavy mountain snows inland from the Cascades to the Sierra. 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 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