Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 PM EST Tue Nov 23 2021 Valid 12Z Fri Nov 26 2021 - 12Z Tue Nov 30 2021 ...Overview... A relatively stagnant upper-level pattern is expected in the medium range period, with mean troughing for the eastern half of the U.S. and quasi-zonal flow shifting to ridging by the weekend across much of the West, except for a southern stream low gradually making its way across northern Mexico and the Southwest as it weakens. This pattern should be relatively dry across much of the contiguous U.S., with the exception of waves of precipitation over the Pacific Northwest through the period, as well as some precipitation coming in across eastern trough, including a round of moderate to heavy lake effect and interior Northeast snow Friday into Saturday. ...Guidance/Predictability Assessment... Guidance continues to show above average agreement on the large scale flow pattern described above particularly through the weekend, and a multi-model deterministic blend of the 00Z ECMWF, CMC, UKMET, and 00Z/06Z GFS was used early in the period. Greater differences arise with energy digging through the western side of the trough serving to reload it. The first shortwave with significant differences between models shows up in the Great Lakes around Sunday, with another possibly diving toward the Northern Plains around Monday or Tuesday. The latter shortwave spins up a closed 500mb low across the Great Lakes region in the 00Z ECMWF by Tuesday; a hint of this is shown by the 00Z GFS and a weaker version is shown by the new 12Z ECMWF. Ensemble guidance shows a bit of a split with GEFS members having more ridging across the north-central U.S. and EC ensemble members showing more troughing. So uncertainty is still high with these features. At the surface, there is still a possibility of a surface low to spin up and deepen over the Atlantic Monday into Tuesday, but the timing and strength of the low vary. Ensemble means are weaker, as expected, with the low but do provide a nice middle ground solution at this time, particularly the 00Z GEFS and EC means that were right on top of each other. Incoming 12Z guidance appears it may be more clustered with this low. The WPC forecast for these features shifted from a deterministic blend early on toward utilizing more of the GEFS and EC mean solutions as the period progressed. Some differences between models also remain with the track and timing of the southern stream mid-upper low tracking northeastward from Baja California into the Southwest. 00/06Z GFS runs linger a closed low longer than other guidance, but all guidance has a general weakening trend as the period progresses. Additionally, the precise orientation of an atmospheric river event this weekend into British Columbia and the Pacific Northwest just north of the low amplitude ridging remains in question--GFS have generally kept the greater integrated water vapor transport confined to western Canada, while the 00Z ECMWF brings it into the Pacific Northwest. This will have an impact on how much rain impacts the Olympics and northern Cascades versus the bulk of the precipitation staying north in British Columbia, increasing uncertainty. For these features, a multi-model blend sufficed to minimize differences and produced a middle ground solution at this point, and will continue to monitor. ...Weather/Hazards Highlights... As the period begins Friday, a cold front exiting the East Coast will bring westerly flow to the Great Lakes region and the Northeast, favorable conditions for lake effect and interior Northeast (particularly higher elevation) snow through Saturday. 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 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 may bring some light precipitation to the region but nothing looks particularly impactful at this time. A building trough over the East should keep temperatures below normal through the entire five day period with the greatest anomalies (-10 to -15F) likely on Friday into Saturday from south Texas/lower Mississippi Valley to the Ohio Valley and Mid-Atlantic. Meanwhile, amplified ridging will promote a warming trend across the West and into the Plains with several days of +10 to +20 anomalies (locally higher) likely, especially across parts of the northern and central High Plains Sunday and Monday. Tate/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