Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Dec 21 2023 Valid 12Z Sun Dec 24 2023 - 12Z Thu Dec 28 2023 ...Heavy Snow Threat Spreads to the North-Central Plains Christmas Eve... ...Heavy Rain Threat from the Lower Mississippi Valley to the Southeast Christmas Eve and Christmas and then the Mid-Atlantic toward Midweek... ...Overview... A closed southern stream upper low off southern California will open and eject northeast to the Colorado Rockies this weekend before phasing with a digging northern stream trough over Colorado to become a multi-phase and slow moving low that shifts slowly downstream through late next week. The weekend translation of the dynamic system will spawn downstream south-central Plains cyclogenesis/frontogenesis with phasing/occlusions keeping eastward motion slow through Monday/Christmas. Return Gulf of Mexico moisture and instability will fuel an emerging area of enhanced rains and convection over the eastern portions of the Southern Plains/Lower MS Valley then Southeast Sunday though Monday/Christmas that then shifts main focus to the Mid-Atlantic and onward 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... The WPC medium range product suite was primarily derived from a composite blend of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with compatible input from the 01 UTC National Blend of Models (NBM) and WPC continuity in a pattern with near average predictability for Christmas Eve and Christmas. The models offer an overall similar mid-larger scale flow/QPF pattern evolution, but show less than stellar smaller scale embedded system amplitude and timing differences in both a southern and more progressive northern stream flows. However, the composite seems to smooth the rough edges consistent with predictability and has overall good ensemble support. The Canadian and UKMET runs gradually become slight outlilers with a bit slower flow progression over the lower 48 into Tuesday, but opted to keep the aforementioned blend then given plausability given history of amplified flow regimes and guidance biases. Newer 00 UTC guidance generally remains in line with the mid-upper level flow evolution, with some trend away from progression, but still offers embedded system detail differences to vary local focus increasingly over time. Acordingly, growing forecast spread and run to run cycle variance growth in the models prompted a quick guidance preference switch to the still compatible GEFS/ECMWF ensemble means at longer time frames into Wednesdaty/next Thursday. ...Weather/Hazards Highlights... Complex upper trough progression and Plains surface system organization supports a heavy snow threat from the southern Rockies through the north-central Plains in time for Christmas Eve and Christmas. The WPC Winter Weather Outlook (WWO) shows rising/enhanced plowable commahead snow probabilities. 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 difluence/right entrance region upper jet dynamics and also from a strong/surging subtropical jet. Maintained the Day 4/Christmas Eve Slight Risk ERO. Expect a heavy rainfall signature from the Southeast/Southern 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. In this period, 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 well upstream, mean upper trough amplitude set 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/long fetch southerly moisture flow should allow for some periods of moderate to heavy precipitation into mainly coastal areas and especially south facing favored terrain from the Pacific Northwest to northern California Sunday night through Tuesday, with additional activity into Wednesday/next Thursday. A WPC ERO Marginal Risk area was introduced for Day5/Christmas when guidance shows the best defined atmospheric river signal into coastal Washington and in particular the Olympics. 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. 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 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