Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Dec 24 2024 Valid 12Z Fri Dec 27 2024 - 12Z Tue Dec 31 2024 ...Unsettled weather for the West Coast with periods of heavy rain and high elevation 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 by next Monday as a relentless series of Pacific systems shoves the downstream ridge along, with the flow pattern evolving into a very broad but climatologically modest upper level trough across a much of southern Canada and the Lower 48 by next Tuesday. This will allow progressive systems to move fairly quickly for the most part, with the main exception being a low pressure system moving north into the Upper Midwest ahead of a broad but negatively tilted upper trough early on. For the most part, this should be a mild pattern for the Lower 48 with much of the country experiencing above normal temperatures. ...Guidance/Predictability Assessment... The latest guidance continues to show fairly good agreement on the large scale pattern, uncertainty in the details remains. The winds, pressures, fronts, QPF, and to some degree the PoPs used a blend of the 12z/18z deterministic guidance early on, before blending in some of the 12z NAEFS/12z ECMWF ensemble means to deal with the detail differences. Other grids relied more heavily on the 01z NBM. Overall, this maintained reasonable continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Multiple rounds of moderate to heavy rainfall are expected to impact the West Coast through the remainder of this week 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. A Marginal Risk of Excessive Rainfall is in effect for coastal Washington, Oregon, and northern California on Friday and Saturday for Friday/Friday night. When combined with the rainfall expected through Friday night, enough of a QPF signal appears to be there through Saturday for a day 5 Slight Risk area for far southwest OR and far northwest CA, which was coordinated with the MFR/Medford OR and EKA/Eureka CA forecast offices. 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 is expected at elevation through Sunday. Tight pressure gradients may result in some periodically gusty winds for some locations as well. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the Plains, Mississippi Valley, and Southeast, which spreads into the East late this weekend into early next week. A widespread thaw is expected across a significant portion of Ohio Valley and the southern Mid-Atlantic states, with highs rising into the 50s to lower 60s for days on end. Locations in the Plains and Midwest will see temperatures 20-25F above average in spots Friday through Monday, with similar anomalies across New England on New Year's Eve. Low temperature anomalies in the North Woods of Minnesota are forecast to be impressive, 30-32F Friday and Saturday, albeit still below the freezing mark since it's late December. Readings closer to climatology are initially expected near the West Coast, but with the broadening but modest upper level trough developing early next week, the area of near average readings will move east from the West Coast into the High Plains into New Year's Eve. Roth/Santorelli 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