Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 AM EST Wed Dec 18 2024 Valid 12Z Sat Dec 21 2024 - 12Z Wed Dec 25 2024 ...Heavy Precipitation/Windy Pattern focus for the Pacific Northwest/Northern California this weekend into the Holidays... ...Overview... Pattern amplification across the CONUS during the medium range period is expected by the weekend as deep troughing becomes established over the East, and a strong ridge builds over the West- Central states. A series of 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 continued eastward progress of the leading Pacific shortwave into the Plains by early next week helping to push the Eastern U.S. upper trough eastward with a return to a more progressive (but still potentially active) overall pattern. ...Guidance/Predictability Assessment... Models continue to show good agreement on the overall pattern evolution during the medium range period, but continued uncertainty in the details and timing of individual systems. The models have a good handle early period on the periodically reinforced Eastern trough, although there is some uncertainty on the amplitude of a shortwave dropping down the west side of the trough through the Midwest and into the Mid-Atlantic. Beyond this, the attention shifts out west as a series of shortwaves progress inland across the West into the Central U.S. during the period. Models agree on the presence of these systems, but some timing and strength questions, which have huge implications on the QPF amounts and coverage along especially the West Coast. As the initial shortwave on Saturday moves inland, it may eventually spin up a brief and compact closed low over/near the Great Lakes early next week, while additional southern stream energy induces a weak and uncertain surface system across the southern Plains. The strongest of the shortwaves looks to enter the West Coast by next Tuesday, with some general agreement southern stream energy may help amplify this trough over the Great Basin or Southwest U.S. by Christmas. The WPC blend for tonight was able to use a nearly equal blend of the ECMWF, GFS, and UKMET (the CMC was more amplified with the Eastern U.S. shortwave this weekend) through Day 5. After Day 5, gradually increased the ensemble means to half the blend percentage to help mitigate the uncertainty with the Western U.S. shortwaves. ...Weather/Hazards Highlights... Following a departing initial shortwave out of the East this weekend, expect lake effect snows to continue in favorable downwind locations of Lakes Michigan, Erie, and Ontario into early next week. The trailing arctic airmass should bring daytime highs down to as much as 15 to 20 degrees below normal for especially parts of the Northeast and close to that in the Mid- Atlantic into early next week, with moderation expected thereafter and just in time for the Christmas holiday. Over the West, upper dynamics and windy surface systems will start to push farther eastward by the weekend, with the first of a series of atmospheric rivers on Saturday and increasing rainfall potential for portions of northern California and mountain snow in the higher terrain of the Cascades and California. This system should be fairly progressive, but modest moisture values and some higher rain rates were enough to continue a marginal risk on the Day 4 Excessive Rainfall Outlook across coastal northern California. This enhanced activity will likely continue into next week as another couple of ARs move through, with the last one possibly stronger than the previous two with higher potential for impactful rainfall early to mid next week. As the leading shortwave reaches the central U.S. by next Monday, precipitation may develop over parts of the Plains/Midwest but with low confidence in specifics at this time. Mean ridging aloft will generally keep temperatures over the interior West and into the Plains above average through the period with broad coverage of plus 10 to 20 degree anomalies. A few daily records (mainly for daytime highs) are possible for the West this weekend. 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