Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Dec 25 2024 Valid 12Z Sat Dec 28 2024 - 12Z Wed Jan 01 2025 ...Unsettled weather for the West Coast with periods of heavy rain and high elevation mountain snow... ...Overview... The large scale flow pattern will feature a rapid succession of Pacific systems moving into/through the West and feeding into a mean upper trough of moderate depth aligned over the Plains/Mississippi Valley, while an upper ridge initially near the East Coast drifts into the western Atlantic. This pattern will lead to areas of heavy precipitation along the northern-central West Coast through the weekend and snow over the northern half or so of the Rockies, along with some areas of strong winds. In addition a broad area of precipitation (mostly rain) will spread across the eastern half of the country with some enhanced totals possible. For the most part, this should be a mild pattern for the Lower 48 with much of the country experiencing above normal temperatures initially, which drop back toward climatology by mid next week. ...Guidance/Predictability Assessment... The guidance is showing better agreement than last night, but detail issues remain. A composite of 12Z/18Z operational model guidance provided a good representation of consensus or intermediate ideas for the first half of the period, with some 12Z NAEFS/12Z ECMWF mean guidance incorporated thereafter to account for increasing detail uncertainties. ...Weather/Hazards Highlights... Multiple rounds of moderate to heavy rainfall will likely impact the West Coast through the remainder of this week and into the weekend as multiple frontal systems approach the coast, with the signal for heavy rainfall becoming strongest on Saturday concentrating more near the California/Oregon border with time. Flooding will be possible in rivers and streams, with more flashy responses possible where burn scars exist and possibly near the steep terrain of the coastal mountain ranges and into the foothills of the Cascades. The combination of rainfall expected through Friday night and a more concentrated heavy QPF signal in some guidance focused over far southwest OR and far Northwest CA on Saturday continues to favor a Day 4/Saturday Slight Risk area over that region. While QPF is less the following day, what should be increased sensitivity allowed for the introduction of a Day 5/Sunday Marginal Risk for western OR and northwest CA. Moisture spreading inland across the West will likely produce heavy snow over the higher/highest terrain of the Cascades and Rockies, as well as portions of the Sierra Nevada. Expect the heavy snow potential to continue through Sunday. Precipitation over the Pacific Northwest should trend lighter early next week, with Rockies snow tapering off by Tuesday. Tight pressure gradients may result in some periodically gusty winds for some locations over the Pacific Northwest, Interior West, plus the northern/central Rockies and High Plains. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the Plains, Mississippi Valley, and East. Some moderate to heavy rainfall will be possible within a general area from the lower half of the Mississippi Valley into the Mid-Atlantic and Northeast. The wetness of the some most recent guidance in and near TN and the southernmost Appalachians showing local amounts in the 3-5" range led to the introduction of a Day 4/Saturday Marginal Risk area, which shifts into the vicinity of Pennsylvania on Day 5/Sunday. Any wintry weather should be limited to locations close to the Canadian border. While day-to-day trends in temperature were noticeably downward in portions of the Midwest, Ohio Valley, and southern Mid-Atlantic states, a majority of the central/eastern U.S. should see well above normal temperatures from late this week into early next week. Expect the warmest anomalies to be for morning lows across the east-central part of the country Friday-Sunday, in the plus 20-30F range and possibly challenging daily records for warm lows in some areas. Warmest min temperature anomalies should be more confined to the Upper Mississippi Valley to New England early next week. Also the southern Plains could see highs reach 15-25F above normal on Monday in the warm sector ahead of a system reaching the High Plains by early in the day. Readings closer to climatology will broaden eastward from the West Coast early next week, reaching the Plains early next week and the East by mid-next week. Roth/Rausch 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