Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 PM 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 coming inland across southern California or far northern Mexico around next Saturday. This likely will bring periods of enhanced precipitation onshore across California and then increasingly inland heading into next weekend. Meanwhile, a deep upper low/trough over the East Coast will cut off another low which lifts north well off the East Coast next week. In between the Western and Eastern features, upper riding will build across the Central into Eastern U.S.. ...Guidance/Predictability Assessment... Outside of the 00z UKMET, the remainder of the models and ensembles showed fairly good clustering for the upper low which lifts well off the East Coast mid to later next week. The UKMET seemed much too close to the coast and was not included in the blend today. In the West, the outlier for the deep low dropping southward along the coast was the 06z GFS which shifted noticeably westward with the low placement, resulting in being slower to bring precipitation onshore across central and southern California. Interesting to note though that the 12z GFS came in just slightly east of it's 06z position, while the 12z ECMWF was slightly west, and the UKMET and CMC are still closest to the coast. So there remains some run to run variability and wobble with this low. Through the 00z/06z guidance, the WPC forecast leaned more heavily on the ECMWF and the CMC early in the period due to the GFS concerns off the West Coast. By day 5, and especially beyond, used increasingly more of the ensemble means as overall spread generally increased. This approach maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Expect lingering wrap back snows in cold post-frontal flow to the lee of the lower Great Lakes and more broadly across especially favored terrain/mountains of the Northeast into Tuesday in the wake of the deep coastal storm, secondary low genesis and deep troughing aloft. Out West...precipitation chances will be increasing next week down across the West Coast, as a plume of deepened moisture is channeled around a closed upper trough/surface system digging southward over the eastern Pacific. Timing and exact amounts remain in question are are depending on how close to the coast the upper low actually tracks, however rain amounts and rates could still be sufficient to cause some local flooding concerns, especially given some instability could reach coastal areas. The main focus of the 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 continue for Tuesday and Wednesday from northern to central coastal California. Organized enhanced activity will work into Southern California 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 along the East Coast may begin moderately below normal on Tuesday in the wake of the deep lead storm, with frost and freeze conditions extending into parts of northern Florida Tuesday/Wednesday. Conditions should trend warmer and back towards normal for the rest of next week/weekend though. 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. 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