Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM EST Mon Dec 23 2024 Valid 12Z Thu Dec 26 2024 - 12Z Mon Dec 30 2024 ...Unsettled weather for the West Coast with periods of heavy rain and mountain snow... ...General Overview... The general flow pattern will deamplify/become more zonal with time as ridging in the East moves into the western Atlantic as a relentless series of Pacific systems shoves the downstream ridge along. This will allow progressive systems to move fairly quickly for the most part, with the main exception being to low pressure system moving north from TX into the Upper Midwest ahead of a broad but negatively tilted upper trough. For the most part, this should be a mild pattern for the Lower 48. ...Guidance/Predictability Assessment... The guidance shows small to modest differences early on, driven by the 12z ECMWF wanting to move an upper low and upper trough through the mean ridge. Even within the ECMWF guidance, there's a mixed signal for a preference for the closed system it advertises across the Northeast on Thursday morning -- the apex of the ridge is far enough north to support a progressive mid-level low around that location, but the location and intensity of the mid-level positive anomaly appears too close by. Since it can't be ruled out completely, the preference used for the 500 hPa heights, fronts/pressures/QPF/PoPs leans towards a deterministic blend of the 12z runs of the UKMET, ECMWF, Canadian and the 18z GFS. Later on, some 12z ECMWF/NAEFS mean was used to account for strength and position uncertainty. The remainder of the grids stay closer to the 01z NBM. ...Weather/Hazards Highlights... Multiple rounds of moderate to heavy rainfall are expected to impact the West Coast later this week as multiple frontal systems approach the coast south of lows headed for southwest Canada, with the signal for heavy rainfall becoming stronger by the day between Thursday and next Sunday and eventually concentrating near the California/Oregon border. Flooding will be possible, especially where burn scars exist and possibly near the steep terrain of the Olympics and coastal mountain ranges and into the foothills of the Cascades. A Marginal Risk of Excessive Rainfall is in effect for coastal Washington, Oregon, and northern California on Thursday and Friday. Moisture spreading inland in the West will likely produce heavy snow over the higher/highest terrain of the Cascades and Rocky mountains, and portions of the Sierra Nevada. Heavy snow will focus over the Northwest on Christmas and expand through the Intermountain West/Rockies later this week. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the Plains, Mississippi Valley, and Southeast. Southerly flow will bring moisture up from the Gulf of Mexico into the Ark-La-Tex region where multiple rounds of rain are expected. A Maringal Risk of Excessive Rainfall exists for that area on Thursday/Thursday night. Widespread above normal/mild temperatures by late December standards are expected across the Central U.S. for the entire forecast period, ranging from 5 to 20 degrees above average. Relative mildness spreads into the East with time. 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 the West Coast. Roth/Dolan 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