Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Wed Nov 24 2021 Valid 12Z Sat Nov 27 2021 - 12Z Wed Dec 01 2021 ...Overview... The upper level pattern this weekend into early next week is forecast to be amplified and fairly stagnant with troughing in the East and ridging in the West, with the exception of southern stream energy tracking across northern Mexico and Texas while weakening and eventually getting absorbed into the Eastern trough. A couple of shortwaves should initially help reinforce the Eastern trough, while by Tuesday-Wednesday additional shortwaves coming into the north-central U.S. may flatten the overall upper pattern a bit, but with some uncertainty. ...Guidance/Predictability Assessment... Guidance continues to show reasonably good agreement for the large scale flow pattern but offers plenty of uncertainty still in the details. Energy reloading through the western side of the trough in the East on Sunday-Monday continues to show slight timing and magnitude differences, which affects a surface low deepening off the Northeast coast. For the 00/06Z model cycle, the ensemble means clustered well with the surface low position around day 5/Monday, and were aligned most closely to the 00Z ECMWF and CMC and 06Z GFS, so the WPC forecast mainly favored this position, which was close to continuity. The 00Z GFS and UKMET were farther west/closer to the coast comparatively, and the new 12Z GFS is now showing a double barreled low situation with a low closer to the coast and one to the east, while the 12Z GEFS mean is slightly westward. Meanwhile the incoming 12Z ECWMF is farther south (around the latitude of the NC/VA border) with its low position Monday morning before jumping north toward other guidance, again showing uncertainty. The placement of this surface low will have impacts for the Northeast including how much precipitation falls, so this will continue to be monitored. Despite some differences, consensus now shows troughing in the East lifting somewhat on Tuesday into Wednesday. This occurs as shortwave energy from central Canada that in some prior model runs served to strengthen the preexisting trough, as well as a shortwave coming into the Pacific Northwest around day 5/Monday and propagating downstream, now for the most part appear to stay separated from the Eastern troughing. Differences certainly remain with the timing and position of these shortwaves. WPC's blend for this area and for the latter half of the medium range period consisted of the deterministic 06Z GFS, ECWMF, and CMC as well as the GEFS and EC mean, which admittedly washes out some shortwaves but seems to be the best bet to smooth out differences considering run-to-run variability. ...Weather/Hazards Highlights... A plume of moisture/atmospheric river is forecast to take aim at western Canada and into Washington on Saturday, north of a low amplitude ridge. Precipitation should remain mostly rain even in higher elevations for this event due to the warm temperatures, with heaviest totals likely along favorable terrain. Lighter precipitation is forecast to continue in the Pacific Northwest and into the Northern Rockies through early next week, before another round of heavier precipitation is likely for the Pacific Northwest around Tuesday-Wednesday. Meanwhile, light precipitation is possible with energy moving through the base of the trough for the Midwest/Great Lakes and possibly the Ohio Valley and Mid-Atlantic. Precipitation (including some snow) could be enhanced downwind of the Great Lakes and in higher elevations, and possibly in eastern New England on the backside of the Atlantic surface low. A couple of rounds of light rain are possible in Texas, while much of the rest of the country looks to remain dry. The mean 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. 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 Hazards: - Heavy rain across portions of the Pacific Northwest and the Northern Great Basin, Sat, Nov 27 and Tue-Wed, Nov 30-Dec 1. - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Tue-Wed, Nov 30-Dec 1. - Flooding possible across portions of the Pacific Northwest. - Flooding likely across portions of the Pacific Northwest. - Much below normal temperatures across portions of the Southeast and the Southern Appalachians, Sat, Nov 27. 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