Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Wed Nov 24 2021 Valid 12Z Sat Nov 27 2021 - 12Z Wed Dec 01 2021 ...Overview... The upper level flow during the medium range period should remain mostly stagnant and amplified consisting of mean troughing in the East and ridging out West. A southern stream upper low over Baja California on Saturday should weaken with the leftover energy shifting into the southern Plains and eventually being absorbed by the Eastern U.S. trough. A couple of shortwaves into the Eastern trough should help reinforce it through the period. General ridging should persist out West, though periodically may be interrupted by weak waves of shortwave energy. ...Guidance/Predictability Assessment... Guidance continues to show good agreement on the large scale flow pattern but offer plenty of uncertainty still in the details. Early on, there are some timing/intensity differences with reloading energy into the Eastern trough, which eventually results in some placement differences of a deepening low pressure system off the Northeast coast. The latest GFS is stronger with the system, while the CMC is farthest south. By day 5/Monday, the GFS and CMC are a little bit stronger with a shortwave moving into the Pacific Northwest, which eventually propagates downstream into the Eastern trough, and offers additional differences in evolution/shape of the Eastern trough late period. By that point though, there's enough run to run variability in the models that leaning more on the ensemble means seems to be the best for now. The WPC forecast used a general deterministic model blend for days 3-5 amidst decent model agreement. Thereafter, more weighting towards the ensemble means were used to help mitigate some of the smaller scale, hard to resolve, detail. Did maintain modest amounts of the ECMWF and GFS just for some added system definition. This approach maintains good continuity with the previous WPC forecast for days 3-6. ...Weather/Hazards Highlights... Westerly flow across the still relatively warm Great Lakes on Saturday should provide favorable conditions for lake effect and interior Northeast (particularly higher elevation) snow. Moderate to locally heavy snow amounts downwind of Lake Ontario, the Adirondacks, and northern New England are possible. Meanwhile, a plume of moisture/atmospheric river coming into the Pacific Northwest could bring another round of moderate to locally heavy rains to parts of western Washington, with the heaviest totals likely along favorable terrain. Lighter precipitation should spill over into northern parts of the Rockies as well this weekend but quickly dry out early next week. Then another round of precipitation is forecast for the Pacific Northwest by around next Tuesday. The remainder of the country should remain fairly dry through the weekend and into next week. A weak system moving through the Ohio Valley into the East and energy across the southern Plains may bring some light precipitation to these regions but nothing looks particularly impactful at this time. A building trough over the East should keep temperatures below normal through Monday with some moderation back towards normal on Tuesday and Wednesday. Meanwhile, amplified ridging will promote a warming trend across the West and into the Plains with several days of +15 to +25 anomalies (locally higher) likely, especially across parts of the northern and central Plains Sunday and Monday and again on Wednesday. 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml