Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EST Sun Dec 17 2023 Valid 12Z Wed Dec 20 2023 - 12Z Sun Dec 24 2023 ...Emerging Precipitation Threat for Southern California and the Southwest... ...Overview... A closed upper trough/low will dig southward off California mid-late week before ejecting inland across southern California/northern Mexico and the Southwest next weekend. This will bring periods of enhanced precipitation into southern California, the Southwest. Meanwhile, a deep upper low/trough over the East Coast midweek will cut off another low off the East Coast into later week, acting mainly as a maritime threat. In between the Western and Eastern features, warming upper riding will position over the Central into East-Central U.S.. ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived for Wednesday into Friday from a composite blend of best clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET 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 above normal predictability. The 12 UTC Canadian solution offers an outlier solution whose closer held Atlantic closed low proximity allows for much wetter pattern for New England mid-late week, but the newer 00 UTC run has trended strongly toward the guidance composite. Prefer a composite of the GFS/ECMWF and GEFS/ECMWF ensemble means by next weekend for the best detail as feasible in a period with slowly growing forecast spread. ...Weather/Hazards Highlights... Locally enhanced rain chances will shift southward across California this 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 may be sufficient for some local flooding concerns, especially given some instability could reach coastal areas. The WPC Excessive Rainfall Outlook (ERO) offers Marginal Risks for Wednesday into Thursday from central to southern coastal California. Organized rains and terrain/mountain focusing snows set to work inland through the Sierra and with some threat from the Southwest across the south-central Great Basin/Rockies for the holiday weekend with slow system approach. Meanwhile, system/energy downstream propagation may also generate an emerging wet pattern from the south-central Plains with system genesis and with more uncertainty to the East given return moisture inflow signal in guidance. This could include some northern tier wintry precipitation. Meanwhile upstream, upper shortwave trough and surface system progression into the Northwest should support periods of enhanced precipitation Friday into 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