Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM 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... An overall amplified pattern 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 West to Central states. 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 still potentially active) overall pattern. ...Guidance/Predictability Assessment... Latest models and ensembles continue to show good clustering and predictability on the large scale, but there are some lingering uncertainties on timing and details of individual systems, which may have significant impacts on sensible weather. The Eastern trough should begin to push off the coast Sunday into Monday ahead of a leading shortwave into the northern Plains. The guidance has shown better clustering on this feature but there is some uncertainty in the evolution of this into next week which some models indicating energy may hang back over the Great Lakes rather than progress quickly through the Northeast. A general model blend seemed to serve as a good starting point here. The next shortwave will enter the west by Monday, with amplification expected over the Central Plains before it begins to lift out by Wednesday. The models show good agreement on this system as well, but some question on how quickly it weakens as it lifts into the Midwest. The next system behind this seems to be much stronger than its predecessors with plenty of agreement that this may form a brief closed low over the Southern Plains around next Thursday. But there is still notable uncertainty on its timing and specifics so the late period WPC blend used a modest amount of the ensemble means to help smooth these differences. ...Weather/Hazards Highlights... A series of mostly weak atmospheric rivers will be ongoing by this weekend over 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, so the previous marginal risk we had on the Excessive Rainfall Outlook has been dropped this cycle. By Monday, however, the 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 Monday ERO period, though the activity will continue into Tuesday as well. Model QPFs continue to suggest several inches of rainfall possible, especially for parts of northwest California, with accumulating and possibly heavy snows in the higher terrain of the Cascades and the Sierra. Farther east, the leading shortwave 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 will allow precipitation to develop across the Plains into the Midwest next week, but with lingering uncertainty in the specifics. 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 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