Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 21 2024 Valid 12Z Tue Dec 24 2024 - 12Z Sat Dec 28 2024 ...General Overview... A progressive upper level flow pattern is expected to be in place across the Continental U.S. for the early to middle portion of the week. An energetic storm track across the eastern Pacific will result in multiple fast moving shortwaves crossing the nation through the end of the week and into next weekend, and this will tend to keep a separate northern and southern stream flow. One of these shortwaves crossing Texas will have enough lift and dynamics to support a heavy rainfall event across portions of the central Gulf Coast and Deep South on Tuesday, and heavy rain and mountain snow is likely to continue across portions of the West Coast with some atmospheric river events likely. Most of the nation should have above normal temperatures with no arctic air masses expected. ...Guidance/Predictability Assessment... Given the complexity of the overall weather pattern, the 00Z model guidance suite is in reasonably good agreement for the beginning of the forecast period Tuesday. The GFS becomes stronger with the shortwave exiting the East Coast Christmas morning, and the ECMWF is slower with the southern stream shortwave across eastern Texas. There has been a trend for a stronger trough to develop over the western High Plains by Thursday, with the also a little slower with that. Things become more uncertain for the West Coast with the timing of future Pacific shortwaves towards the end of the week, with the GFS becoming more progressive. This also holds true going into next Saturday as well. Upon examination of the machine learning guidance, there is better support for the CMC and ECMWF solutions by Friday/Saturday, so the GFS was removed from the forecast blend for Day 7. The ensemble means were gradually increased to about half by that time. ...Weather/Hazards Highlights... It still remains the case that moderate to heavy rainfall is expected to persist across portions of northern California and into western Oregon early on Tuesday, with the heaviest rain expected for the western slopes of the Sierra Nevada in central/northern California, where a Marginal Risk of excessive rainfall is valid for the Day 4 outlook, and subsequent rounds of rain and mountain snow appear likely through Christmas with a Day 5 Marginal Risk in effect for western Oregon and Washington, and probably continuing into the end of the week. Heavy snow is expected for the higher terrain of the Cascades and the Sierra, then working inland across the Northwest and north-central Intermountain West/Rockies. The southern stream trough is expected to develop further with potential closed low development on Tuesday as deeper moisture surges 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 remains valid for the WPC Day 4/Tuesday excessive rainfall outlook from eastern Texas through Arkansas. 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 given some QPF placement differences in the model guidance. Some strong thunderstorms may also develop in this same general area. After a cold start to the week, the Eastern U.S. should see a moderation trend in temperatures going into the Christmas holiday as highs return closer to climatological averages. Widespread above normal temperatures will expand from the West into the Plains and Midwest through the majority of the period, with no signs of any arctic air mass incursions with flow originating from the Pacific. Daytime highs in some locations could be 15 to 25 degrees above normal. 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