Extended Forecast Discussion NWS Weather Prediction Center College Park MD 138 PM 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... Not much change to report from the overnight forecast package. Upcoming pattern amplification aloft over 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 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 continued eastward progress of the leading Pacific shortwave into the 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... 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. This generally 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... 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 quite cold 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 and inland over the Northwestern U.S.. 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/Saturday Excessive Rainfall Outlook across coastal northern California. The next dynamic shortwave working inland Day 5/Sunday led to the introduction of another Marginal Risk area spread up through the Pacific Northwest. This enhanced activity will likely continue into next week as another couple of ARs move through, with the last one and main deep low will likley be stronger than the previous two with higher potential for impactful rainfall early to mid next week. This leads into a wet week 2 period as heralded by CPC. 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 10-20 degree anomalies. A few daily records (mainly highs) are possible for the West this weekend. 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