Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Dec 20 2024 Valid 12Z Mon Dec 23 2024 - 12Z Fri Dec 27 2024 ***Multiple rounds of rain and snow expected for the West Coast region next week with an energetic Pacific storm track*** ...General Overview... A quasi-zonal upper level flow pattern is expected to be in place across the Continental U.S. for the beginning of next week once the deep upper trough departs the East Coast. An upper ridge tries to develop across the Intermountain West and Rockies on 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 copious rainfall ahead of it. Looking ahead to the end of the week, an 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 mountain snow. ...Guidance/Predictability Assessment... The 00Z model guidance suite is in good overall synoptic scale agreement for early in the week, although the 00Z CMC is slower with the arrival of a lead shortwave across the Pacific Northwest, and a little weaker with the deeper trough moving in late Tuesday/early Wednesday, and then a little slower by Wednesday night across the Rockies compared to the model consensus. The GFS becomes stronger with the trough departing the Northeast U.S. midweek, but relatively comparable elsewhere. By the end of the week, things become more uncertain with the next storm system across the southwestern U.S., with recent ECMWF runs slower and more amplified than the ensemble means suggest. The majority of the machine learning guidance also favors a faster scenario than the ECMWF, so this model was weighted less by late Thursday into Friday, whilst increasing the ensemble means to about half by this time to account for the increasing uncertainty. ...Weather/Hazards Highlights... Moderate to heavy rainfall is expected 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 the 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 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 mainly the Northwest and the 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, with an overall increase in model and NBM QPF during the Tuesday-Wednesday time period as deeper moisture surges northward from the central Gulf. Given the potential for higher instability and rainfall rates of an inch per hour or greater with the convection that develops, a Marginal Risk will be valid on the new Day 5/Tuesday excessive rainfall outlook from eastern Texas to southern Missouri. Some strong thunderstorms may also develop in this same general area. A departing surface high over the East will continue much below normal temperatures through Monday, with moderation expected going into the Christmas holiday. 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 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