Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM 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... A general model blend consisting of the deterministic 00z ECMWF/CMC/UKMET and 06z GFS were used from day 3-5, due to their relatively similar evolution of the synoptic pattern across the CONUS. The 00z ECE/CMCE and 06z GEFS were incorporated into the blend with some 06z GFS on days 6 and 7. The 06z GEFS had more spread than its 00z run around the Central U.S. trough beginning on day 5. The deterministic 06z GFS was favored over the other operational models in the blend beyond day 5 due to its more reasonable depiction of the mid-level synoptic evolution across the Central U.S.. ...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. Kebede/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