Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Nov 23 2021 Valid 12Z Fri Nov 26 2021 - 12Z Tue Nov 30 2021 ...Overview... The medium range should remain very amplified in the upper levels through the entire period. A closed southern stream low over Baja California looks to linger for a few days before weakening and attempting to lift into the Southwest early next week. Meanwhile, a closed low over the Great Lakes will lift into the Northeast while additional energy from the west should act to reload and amplify the trough over the eastern U.S. by early next week. Upper level ridging will build across the Northwest/Rockies this weekend and early next week as the next system approaches the West Coast by Tuesday. ...Guidance/Predictability Assessment... Latest guidance continues to show above average agreement on the large scale flow through most of the period. The greatest uncertainties in the forecast lie with reinforcing energy into the East this weekend and early next week, where the guidance continues to show varying solutions on the details and evolution of the trough. The models indicate the potential for another surface low to spin up and deepen off the Northeast Coast but again, timing and strength of this low are an issue. The CMC is the farthest north lifting the low towards the northwest across Maine on Tuesday, which the ECMWF and GFS are notably farther south and more offshore (GFS farthest offshore). The run to run variability in the deterministic solutions remains all over the place though so confidence in the specifics is low with this system. The ensemble means are weaker, as expected, with the low but do provide a nice middle ground solution at this time. Elsehwere, there are also timing and amplitude differences with the approaching trough into the West on day 7. WPC used a general model blend for days 3-5, with increasing contributions from the ensemble means by late period. This should help to mitigate differences discussed above. This combination maintains good continuity with the previous WPC forecast for days 3-6. ...Weather/Hazards Highlights... A cold front exiting the East coast as the period begins Friday may continue to bring some light to moderate precipitation to parts of the Northeast as well as the Great Lakes. Favorable westerly flow over the still relatively warmer lakes should result in at least modest lake effect snow accumulations mainly downwind of Lakes Erie and Ontario on Friday into Saturday. Another weak shortwave should help direct a plume of moisture into the Pacific Northwest this weekend, possibly bringing 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 parts of the Northwest and Rockies as well this weekend but quickly dry out early next week. 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. 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