Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM 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, and 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... There remains some considerable differences in placement of a cut off low off the East Coast day 3, though guidance has generally trended slightly closer to the coast. The GFS remains the farthest east with this system. Out West, guidance shows better agreement today on the closed low off the California coast which should shift inland around Friday-Saturday. The bigger forecast question is with additional strong energy into the Pacific Northwest late in the week, which some guidance shows closed low development next weekend over the interior West. The GFS is faster with this system, and the ECMWF is strongest and digs it farthest south. The means are a lot weaker but generally follow their deterministic counterparts. The WPC blend for the updated forecast package today used a general model compromise for dayd 3-4 amidst good agreement. After this, continued a general blend of the GFS, ECMWF, and CMC, but with increasing contributions from the ensemble means to account for the growing spread. This approach maintains good continuity with the previous WPC shift as well. ...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. 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 next weekend. Santorelli/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