Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM EST Sat Dec 16 2023 Valid 12Z Tue Dec 19 2023 - 12Z Sat Dec 23 2023 ...Emerging Precipitation Threat to focus into California and the Southwest next week... ...Overview... A closed upper trough/low remains slated to dig southward off the West Coast next week. Feature proximity may bring periods of moisture enhanced precipitation onshore and then increasingly inland heading into next weekend as it slowly turns towards southern CA and the Southwest. ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived for Tuesday into Thursday 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 to provide sufficient detail consistent with a pattern with seemingly above normal predictability. The 12 UTC Canadian was a more progressive flow outlier, but the newer 00 UTC run has come back into the fold. Opted to switch to a composite of the GFS/ECMWF and their ensemble means later week into next weekend to maintain as much detail as feasible albeit in a period of slowly growing forecast spread. ...Weather/Hazards Highlights... Expect lingering wrapback snows in cold post-frontal flow to the lee of the lower Great Lakes and more broadly across especally favored terrain/mountains of the Northeast into Tuesday in the wake of the exit of a deep coastal storm, secondary low genesis and deep troughing aloft. Out West...precipitation chances will be increasing next week down across the West Coast, especially as a plume of deepened moisture is channeled around a closed upper trough/surface system digging southward over the eastern Pacific. Rain amounts and rates could be sufficient to cause some local flooding concerns, especially given some instability could reach coastal areas. A main focus of a fueling Atmospheric River is expected to gradually shift southward through California next week as the main low digs then eventually shifts eastward toward southern California/Baja and the Southwest. Marginal Risks of Excessive Rainfall are planned for Tuesday and Wednesday from northern to central coastal California. Organized enhanced activity will work into Southern Califirnia and the Southwest later next week/weekend with main system slow approach. Higher elevation snows are also expected along/inland from the West Coast. Meanwhile, system/energy downstream propagation may also generate an emerging wet pattern from the south-central to east-central U.S. given eventual Gulf of Mexico return moisture inflow response. Additionally, there is also a growing guidance signal showing that renewed upper shortwave trough and surface system progression into the Pacific Northwest may allow for a burst of moderate precipitation again by next weekend. Temperatures will trend downwards over the East through the rest of the week in the wake of the deep lead storm. Above average temperatures are meanwhile expected to persist across the West and spread into the Central U.S. for the second half of next week. Temperatures in the Southeast will likely remain below average early-mid next week as a cold front passes through and high pressure builds into the region. Frost and freeze conditions may extend into parts of North Florida. 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