Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EST Mon Dec 18 2023 Valid 12Z Thu Dec 21 2023 - 12Z Mon Dec 25 2023 ...Heavy Rain Threat for Southern California/Arizona into Thursday/Friday... ...Emerging Heavy Rainfall/Convection Threat for the South-Central U.S. over the extended holiday weekend... ...Overview... A closed upper trough/low will dig southeastward off California into later this week before working inland over/just south of southern California and the Southwest Friday into Saturday. This will bring periods of leading enhanced precipitation into southern California and the Southwest. The ejection of the dynamic system is expected to spawn downstream south-central Plains cyclogenesis/frontogenesis next weekend whose lead return moisture and instability could fuel an emerging area of enhanced rains/convection. ...Guidance/Predictability Assessment... The WPC medium range product suite was mainly derived for Thursday into Saturday via a composite of best clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/Canadian along with compatible input from the 01 UTC National Blend of Models (NBM) and WPC continuity in an effort to provide sufficient detail consistent with a pattern with seemingly near normal predictability. The closer proximity of an alternate 12 UTC UKMET cutoff coastal low solution holds in a wetter pattern than most guidance for eastern New England in this period, but the newer 00 UTC run has trended offshore. Later weekend/Monday, a blend of the ECMWF/Canadian and GEFS/ECMWF ensemble means seems to provide best feasible detail in a period with growing forecast spread. The 18 UTC GFS seems a less likely outlier solution at these longer time frames in showing much less upper trough digging into the West than most other guidance. Deep systems and associated flow amplitudes upstream over the Pacific seem to better support upper trough digging into the West for this period and now the latest 00 UTC GFS has flipped to this preferred solution more in line with other 12/00 UTC cycle guidance. ...Weather/Hazards Highlights... Locally enhanced rain chances will work across southern California later week as a plume of deepened moisture is channeled around a closed upper trough/surface system dug over the eastern Pacific. Rain amounts and rates seem sufficient for local flooding concerns, especially given some instability could reach coastal areas. The WPC Excessive Rainfall Outlook (ERO) will offer a collaborated upgrade to a Slight Risk for Day 4/Thursday for southern coastal California. Organized rains and mountain snows are also set to work inland into the Southwest and the south-central Great Basin/Rockies heading into the weekend with system approach/ejection. A Marginal Risk ERO is planned from the California deserts to Arizona for Day 5/Friday. Meanwhile, 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 MS 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 amounts up though the Plains/Mississippi Valley/Midwest, now with a more blocky eastern U.S. surface ridge holding downstream. This could include some northern tier wintry precipitation. 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. 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