Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Fri Dec 20 2024 Valid 12Z Mon Dec 23 2024 - 12Z Fri Dec 27 2024 ...Heavy rain/mountain snow and high wind threat pattern from north-central California to the Northwest into the holidays... ...General Overview... A less amplified/transitional upper level flow pattern with multiple emerging streams is expected over the Continental U.S. by next week once a deep upper trough departs the East Coast and amplified Pacific systems progress inland. An upper ridge tries to develop across the Intermountain West and Rockies Tuesday before a potent Pacific shortwave moves in and produces an atmospheric river event for the West Coast, and another disturbance develops over the Southern Plains midweek and spurs a new surface low with enhanced rainfall in advance. Looking ahead to the end of the next week, a continued periodic and energetic storm track from the Pacific leads to an additional storm system moving inland across the Western U.S. with another round of heavy rain and windy mountain snow. ...Guidance/Predictability Assessment... Forecast clustering and uncertainty have marginally increased over medium range time scales with introduction of a more transitional flow pattern next week, but in particular by later next holiday week. The WPC product suite and collaborated threat messaging was primarily derived from a preferred composite of best clustered guidance from the 06 UTC GFS/GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean. This composite provides max consistency with WPC continuity and the 13 UTC National Blend of Models. This blend process tends to mitigate model variances consistent with predictability. A similar blend of 12 UTC guidance seems in line. ...Weather/Hazards Highlights... It still remains generally the case that moderate to heavy rainfall is expected to persist across portions of northern California and into western Oregon on Monday, when a core axis of the Eastern Pacific trough should push eastward with an increase in the moisture and upslope flow across windward terrain. Moisture and instability with this atmospheric river continues to look sufficient enough for a continued Marginal Risk on the WPC Excessive Rainfall Outlook for Day 4/Monday and to a lesser extent for Day 5/Tuesday, and subsequent rounds of rain and mountain snow appear likely through Christmas and into the end of the week. Model QPFs continue to suggest enhanced rainfall possible over the Pacific Northwest, but especially for parts of northwest California, with accumulating and possible heavy snow focus in the higher terrain of the Cascades and the Sierra, then working inland across the Northwest and north-central Intermountain West/Rockies. Farther east, the leading shortwave and frontal low into the Northern Plains will spread some mainly light rain and snow across portions of the Upper Midwest and the Great Lakes into Monday. The southern stream trough is expected to develop further with potential closed low development in emerging southern stream flow 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 is in place for the WPC Day 5/Tuesday excessive rainfall outlook from eastern Texas through Arkansas. Some strong thunderstorms may also develop in this same general area as per SPC. A departing surface high over the East will continue much below normal temperatures through Monday, with moderation expected going into later next week. Above normal temperatures will expand from the West into the Plains and Midwest through the period. Daytime highs in some locations could be 10 to 20 degrees above normal. Hamrick/Schichtel 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