Extended Forecast Discussion NWS Weather Prediction Center College Park MD 101 PM EST Thu Dec 19 2024 Valid 12Z Sun Dec 22 2024 - 12Z Thu Dec 26 2024 ...Heavy Precipitation/Windy Pattern focus for the Pacific Northwest/Northern California this weekend into the Holidays... ...Overview... A moderately amplified flow pattern over the CONUS is expected to be in place by the start of the medium range period on Sunday with deep troughing over the East and mean ridging over the Western to Central U.S.. A series of dynamic shortwaves originating from amplified East Pacific troughing will push into/through the mean ridge out west, bringing a couple of atmospheric rivers and heavy rain/mountain snows to parts of the West Coast well into the upcoming Holiday week. Current forecasts show eastward progress of a leading shortwave through the Northern Plains and onward by early next week to help push the Eastern U.S. upper trough eastward with a return to a more progressive, but potentially still active flow. ...Guidance/Predictability Assessment... Model forecast clustering and predictability seem much better than average for both the larger scale flow evolution and many embedded systems Days 3-7. A preferred composite of the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian models was used for WPC product creation for Days 3-7. It should be noted that recently the dayshift available model solutions initialized at 00/06 UTC have been slightly better clustered than their respective overnight versions. In either case a model composite has recently seemed consistent with WPC and guidance continuity along with the 13 UTC National Blend of Models. There is also good multi-model ensemble and machine learning model support. This blend process tends to mitigate lingering smaller scale model variances as consistent with individual system uncertainties. Latest information from the newer 12 UTC guidance suite so far remains well in line with this overall forecast plan. ...Weather/Hazards Highlights... It generally remains the case that a series of weak-moderate atmospheric rivers will be ongoing by this weekend into the West bringing a prolonged period of windy and generally unsettled conditions along the West Coast. For Sunday, guidance continues to show some rainfall, but rainfall rates may not be high enough to cause any widespread hazards. By Monday however, a core axis of the Eastern Pacific trough should push east with an increase in the moisture and heavy rain/mountain snow potential. Moisture and instability with this AR looks more sufficient for at least a Marginal Risk on the WPC Excessive Rainfall Outlook for Day 5/Monday, with this type of to enhanced activity slated to periodically continue into the holidays, with cumulative impact potential to monitor. 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 weak rain or snow across portions of the Upper Midwest and the Great Lakes into Monday. The southern stream system may close, with moderate cyclogenesis/frontogenesis leading to some precipitation development across the Plains into the Midwest next week, but with lingering specifics uncertainty. A departing surface high over the East will continue much below normal temperatures through Monday, with moderation thereafter. 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. Santorelli/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