Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Sun Dec 22 2024 Valid 12Z Wed Dec 25 2024 - 12Z Sun Dec 29 2024 ***Unsettled weather for the West Coast with periods of heavy rain and mountain snow*** ...General Overview... A progressive and split-flow upper level pattern will be in place across the nation with multiple shortwaves crossing the Rockies and then the Plains. The storm system reaching the West Coast Tuesday is expected to reach the western High Plains by Thursday and potentially form a closed upper low, and this will likely be followed by another well developed shortwave trough that will reach the Midwest states by early Sunday. Meanwhile, an upper ridge likely develops near the East Coast, and continued strong onshore flow across northern California to western Washington from the Pacific, with multiple disturbances moving in from an energetic storm track, will keep daily rain and snow chances in place with some atmospheric river events likely. ...Guidance/Predictability Assessment... At the time of fronts/pressures generation, the available guidance from the existing 12Z/18Z guidance was in less than stellar agreement across the domain for Christmas day into the end of the week. The main difference was with the 12/18Z GFS runs across the central U.S., depicting a much more progressive southern stream shortwave near the Gulf Coast on Christmas, and well to the north of the ECMWF/CMC solutions across the Midwest on Friday with the next system approaching. Upon examination of the machine learning guidance suite, there was stronger support for a non-GFS compromise for the Wednesday through Friday time period. The 00Z GFS is a little closer to the consensus compared to earlier runs. Heading into next weekend, the overall pattern consolidates into a more amplified one with a deeper synoptic scale trough over the Plains and a ridge axis over the East Coast states. The GFS falls more in line with the overall consensus by this time, and thus incorporated back into the blend for days 6 and 7, with a little more confidence compared to earlier in the forecast period. The ensemble means were gradually increased to about 40-50% by next Sunday. ...Weather/Hazards Highlights... It still remains the case that multiple rounds of moderate to heavy rainfall are expected to persist across portions of northern California and into western Oregon and western Washington through the end of the week, with the heaviest rain expected for the terrain of northwestern California and the coastal range of Oregon. Several inches of rainfall is likely for the combined Day 4-5 time period, and Marginal Risk areas are valid for both of these days. Additional heavy rain is expected going into next weekend as well. Heavy snow is expected for the higher terrain of the Cascades and the Sierra Nevada, then working inland across the Northwest and north-central Intermountain West/Rockies. The second southern stream trough is expected to develop further with potential closed low development on Thursday across the southern Plains as moisture returns northward from the Gulf of Mexico. Given the potential for higher instability and rainfall rates of an inch per hour or greater with the convection that develops, a Marginal Risk is planned valid for the WPC Day 5/Thursday excessive rainfall outlook from the ArkLaTex region to southern Missouri. There is the potential that some of this outlook area could eventually need a Slight Risk as the event enters the short range period, but for now will keep it as a Marginal Risk. Some strong thunderstorms may also develop in this same general area. Widespread above normal temperatures by late December standards are expected across basically all of the Central U.S. for the entire forecast period, ranging from 5 to 20 degrees above average. The greatest positive anomalies are likely across portions of the Upper Midwest and extending into the Ohio Valley by Saturday. Readings closer to climatology are expected near both the West Coast and the East Coast, and no arctic air mass incursions are expected over the next week. Hamrick 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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