Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 AM EST Wed Dec 20 2023 Valid 12Z Sat Dec 23 2023 - 12Z Wed Dec 27 2023 ...Heavy Rain Threat for the South-Central U.S. into the holidays then works to the Southeast/Mid-Atlantic... ...Heavy Snow Threat from the Great Basin/Rockies to the North-Central Plains this weekend into Christmas... ...Overview... A closed upper trough/low will dig southeastward off California before working inland through Baja to the Southwest by Saturday. This will bring periods of leading/wrapping enhanced moisture and precipitation into the Southwest. The ejection of the dynamic system will spawn downstream south-central Plains holiday cyclogenesis/frontogenesis. Return moisture and instability will fuel an emerging area of enhanced rains and convection with broad focus over the South-Central U.S. states. Meanwhile, a threat of heavy snow is expected to develop on the backside of the system from the Great Basin/Rockies into Saturday to the north-central Plains in time for Christmas Eve and Christmas. ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/Canadian along with variance smoothing GEFS/ECMWF ensemble means and input from the 01 UTC National Blend of Models (NBM) and WPC continuity in a pattern with average to slightly above normal predictability. Newer 00 UTC guidance generally remains in line with the mid-upper level flow evolution, but still offers embedded system detail differences to vary local focus increasingly over time. ...Weather/Hazards Highlights... Lingering anomolous moisture to fuel rains a terrain/mountain snow focus and some locally higher winds will work out through the Southwest and the Great Basin/Rockies Saturday with closed southern stream system passage and interaction with the digging of less certain northern stream upper trough energies. However, no Day 4/Saturday Excessive Rainfall Outlook (ERO) threat area has been introduced. Howver, the WPC Winter Weather Outlook (WWO) shows broad enhanced snow probabilities. Upper trough progression and Plains surface system organization does then look to support a heavy snow threat from the central Rockies through the north-central Plains in time for Christmas Eve and Christmas. While no Day 4/Saturday ERO area was depicted for an emergening downsteam rain/convection pattern over the Southern Plains with initial cyclo/frontogenesis and return moisture/instability flow, there is some potential for some locally heavier downpours to monitor as supported by varied models and ensemble probabilities. However, 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/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. This prompted a Day 5/Christmas Eve Slight Risk ERO given runoff risks. Expect a heavy rainfall signature to monitor into longer time frames may extend from the Southeast/Appalachians to the Mid-Atlantic Christmas into Tuesday given system translation 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 Great Lakes to northern New England. 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 proximmity 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 through the holiday weekend into next week. 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