Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Dec 19 2023 Valid 12Z Fri Dec 22 2023 - 12Z Tue Dec 26 2023 ...Heavy Rain Threat for Arizona Friday emerges over the South-Central U.S. through the Holidays... ...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 valid Friday into Christmas was primarily derived from a blend of well clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET along with very compatible input from the 01 UTC National Blend of Models (NBM) and WPC continuity in a pattern with seemingly above near normal predictability. Forecast spread grows slower than normal into longer time frames, bolstering forecast confidence, but added into the WPC forecast composite some input from GEFS/ECMWF ensembles to smooth the rough edges. 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. ...Weather/Hazards Highlights... Organized rains and terrain/mountain snows are set to work inland into the Southwest and the south-central Great Basin/Rockies heading into the weekend with closed system approach and passage. A WPC Excessive Rainfall Outlook (ERO) Marginal Risk area remains in place from the California deserts to Arizona for Day 4/Friday on the warmer/rainy side of the system. Again considered a Slight Risk for parts of Arizona given the precipitable water anomalies look to be well over the 90th if not 95th percentile for this time of year and the ample divergence aloft with the upper low, but with somewhat dry antecedent conditions and a seemingly lack of notable instability. Then, system/energy downstream propagation is slated to generate an emerging wet pattern with potential for some strong convection for the holiday weekend from the south-central Plains into the Lower Mississippi Valley and central Gulf Coast states to monitor with system genesis given return moisture inflow signal in guidance and upper support. The northward expansion of the broad precipitation shield with frontal wave propagation is likely to spread moderate precipitation amounts up though the Plains/Mississippi Valley/Midwest, with a more blocky eastern U.S. surface ridge holding downstream. This could include some northern tier wintry precipitation to include a threat for meaningful snow for the north-central High Plains. Temperatures should trend quite warm across the central U.S., especially into the Northern Plains and Upper Midwest next 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 later this week and especially 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