Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EST Wed Dec 20 2023 Valid 12Z Sat Dec 23 2023 - 12Z Wed Dec 27 2023 ...Heavy Snow Threat Spreads onto the North-Central Plains Sunday... ...Heavy Rain Threat for the Lower Mississippi Valley into the Southeast Sunday and Christmas Monday and the Mid-Atlantic toward Midweek... ...Overview... A closed southern stream upper low off southern California will open and eject northeast to the Colorado Rockies Saturday before phasing with a digging northern stream trough over Colorado Sunday that becomes a multi-phase, slow moving low that shifts east through late next week. The ejection of the dynamic system will spawn downstream south-central Plains cyclogenesis/frontogenesis Saturday night with phasing/occlusions keeping eastward motion slow through Monday/Christmas. Return Gulf moisture and instability will fuel an emerging area of enhanced rains and convection over the eastern portions of the Southern Plains/Lower MS Valley Sunday though Monday/Christmas that then shifts to the Midwest/Mid-Atlantic through midweek. Meanwhile, the threat of heavy snow is expected to shift from the southern Rockies to the north-central Plains Saturday night into Monday/Christmas with the lingering/re-phasing low causing more snow concerns on the back side over the central Plains to Upper Midwest into midweek. ...Guidance/Predictability Assessment... Model preferences were based on the evolution of the wave/redeveloping multi-phase low spreading from the Southwest and east through the medium range forecast time. The ECMWF/CMC/UKMET favor a slower, deeper low crossing the Plains early next week as opposed to the more progressive GFS. This is true for both the 00Z and 12Z runs of these models. Preference is heaviest toward the ECMWF with some CMC/UKMET inclusion and little with the GFS through Day 5. Beyond, the ECENS mean is preferred with some CMC ensemble means included. QPF in the medium range includes enhancements to the 13Z NBM from the 00Z ECMWF with some UKMET/CMC as available. Much uncertainty remains with the strength and position of comma head precip over the central US, so for this shift the 00Z ECMWF was chosen as the best/middle of the road solution. ...Weather/Hazards Highlights... Anomalous moisture and forcing shifts from the Desert Southwest Saturday with additional lower elevation rainfall generally ending shortly after 12Z Sat, so there continues to be no Day 4/Saturday Excessive Rainfall Outlook (ERO). However, the WPC Winter Weather Outlook (WWO) shows broad enhanced snow probabilities. Upper trough progression and Plains surface system organization that supports a heavy snow threat from the southern Rockies through the north-central Plains in time for Christmas Eve and Christmas. System/energy downstream propagation will generate an emerging wet pattern with potential for strong convection set to erupt Christmas Eve with focus from eastern portions of the southern Plains through the Lower Mississippi Valley and central Gulf Coast. Increasing instability with upwards of a 50kt LLJ and 2 SD above normal PWs will combine with favorable upper diffluence/right entrance region upper jet dynamics and also from a strong/surging subtropical jet. With the ECMWF focus expanding north, the Day 5/Christmas Eve Slight Risk ERO is expanded north a bit through much of Arkansas/the Ozarks given runoff risks. Expect a heavy rainfall signature from the Southeast/Appalachians to the Mid-Atlantic Christmas into Tuesday given system translation/re-phasing and added Atlantic moisture. The northward expansion of the broad precipitation shield with frontal wave propagation is also likely to meanwhile spread moderate precipitation amounts up from the Midwest through the Northeast with steady erosion of a leading/cooled eastern U.S. surface ridge. This should equate to some northern tier wintry precipitation from the Central Plains to Great Lakes to northern New England though uncertainty remains given the varied timing of the deterministic solutions. Meanwhile upstream, mean upper trough amplitude to persist over the eastern Pacific should prove slow to translate toward the West Coast, but system intensities and gradually closer proximity suggests that lead flow impulses and deep layered southerly flow should allow for periods of light to moderate precipitation into mainly coastal areas and favored terrain from the Pacific Northwest to northern California Sunday night through Tuesday. Temperatures should trend quite warm across the central U.S., especially into the Northern Plains and Upper Midwest this weekend. Daytime highs could be 20 to 25 degrees above normal in this region, while lows of 25-35 degrees above average are expected. A few record highs are forecast while record warm minimum temperatures may be widespread. Elsewhere, daytime high anomalies return back to normal across the East as upper ridging builds and upper troughing into the West should result in a cooling trend by the weekend. Jackson 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw