Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Dec 17 2024 Valid 12Z Fri Dec 20 2024 - 12Z Tue Dec 24 2024 ...Overview... Expect the upper pattern to become more amplified during the late week through weekend time frame as mean troughing becomes established over the East and a strong ridge builds over the West. A shortwave and its surface system should bring snow potential across the Great Lakes and a fresh blast of arctic air to the East Friday into the weekend. Trailing energy will likely reinforce the trough again during the weekend. In contrast, the West will be quite warm due to the mean ridge aloft. A series of shortwaves will push into/through the mean ridge, with a couple of atmospheric rivers likely bringing a period of heavy rain/mountain snows to parts of the West Coast. 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. ...Guidance/Predictability Assessment... Guidance shows good agreement on the overall pattern evolution during the medium range period but plenty of uncertainty in the timing and details of individual systems. The reinforced Eastern U.S. trough through the weekend shows good enough agreement for a general deterministic model blend. Out West, guidance is in agreement the first system should push onshore around Saturday, but there is plenty of uncertainty in the QPF details and amounts associated with this system. This system should push inland across the Rockies and into the Northern Plains early next week, but disagreement on the timing and track of this system exists. A second weaker shortwave will break into the ridge Sunday into Monday, with a stronger more amplified trough reaching the West Coast around Tuesday, but with uncertainty in the timing of impacts. Given the late period uncertainty, WPC used increasingly more of the ensemble means in the blend, with modest amounts of the ECMWF for some added system definition. ...Weather/Hazards Highlights... Following a departing initial shortwave out of the East, and subsequent systems, expect lake effect snows to continue in favorable downwind locations of Lakes Michigan, Erie, and Ontario. With light snow possible across portions of the interior Northeast as well. By next weekend 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. Over the West, upper dynamics and surface systems will start to push farther eastward by next 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 warrant a marginal risk on the Day 5 Excessive Rainfall Outlook across coastal northern California. This enhanced activity will likely continue through next Monday as another couple of ARs move through, with the last one possibly stronger than the previous two with higher potential for impactful rainfall. 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 West into High Plains above average through the period with broad coverage of plus 10 to 20 degree anomalies. A few daily records (mainly for daytime highs) will be possible over the West late this week. 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