Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Nov 25 2021 Valid 12Z Sun Nov 28 2021 - 12Z Thu Dec 02 2021 ...Overview and Guidance/Predictability Assessment... Mean troughing in the East will be reinforced several times by shortwaves and maintained throughout much of the medium range period (Sunday-Thursday). Elsewhere, an amplified ridge across the West should weaken as a couple of shortwaves move through and into the central U.S., which may also help to flatten the overall upper pattern some by mid to later next week. The guidance continues to show reasonably good agreement for this 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 some slight timing and magnitude differences, which would affect a surface low deepening off the Northeast coast. The 00Z/06Z model cycle trended a bit eastward with the surface low away from the coast compared to the previous model cycle and WPC forecast, which was generally maintained with the incoming 12Z models. Guidance was fairly well clustered with the low position with the main exception of the UKMET, which has been slower and farther south. Shortwaves moving through the trough in the East and their potential spinups into closed lows within the mean trough, as well as the fate of shortwaves tracking through the eastern Pacific, become less agreeable in deterministic guidance as the period progresses from Tuesday onward, with no particularly clear solution at this point. Though one seemingly clear outlier was the 00Z CMC, which showed a strong shortwave moving into the West diving south to meet up with energy over northern Mexico to create a closed low differently than consensus. Thus, the WPC forecast began with a blend of the 00Z/06Z GFS, ECMWF, and CMC, gradually transitioning to a blend that favored the GEFS and EC ensemble means (along with some component of the EC and GFS) by around midweek to minimize individual model differences. Overall this maintained good continuity with the previous forecast, with the main change the aforementioned shift of the Atlantic low and its associated precipitation a bit eastward for this cycle, but will continue to be monitored. ...Weather/Hazards Highlights... The atmospheric river into the Pacific Northwest should be weakening by Sunday, though some lingering moderate to locally heavy rainfall will remain possible across the Olympic Peninsula and far northern Cascades. Relatively light precipitation is likely to continue there through Monday before another increase in moisture brings another round of moderate to locally heavy precipitation to western Washington around Tuesday-Wednesday, which may be mostly rain but with some uncertainty in snow levels at this point. Light precipitation should spill into the Northern Rockies as well. Northwesterly flow across the Great Lakes should continue to support periodic lake effect/enhanced snowfall while upper level-energy brings waves of light precipitation across the Midwest to interior Northeast. Downwind of Lake Erie is expected to be a focus for lake effect snow Sunday-Monday while Lake Ontario could be a focus later in the week. Given the eastern trend of the Atlantic low for this model cycle, less precipitation is shown for New England early in the week, but light rain and snow could still wrap around the low onshore. Elsewhere, the contiguous U.S. should remain mostly dry, other than some early week showers for Florida and increasing moisture for Texas midweek onward. The mean trough over the East should keep temperatures slightly below normal into Tuesday with some moderation back towards normal Wednesday and Thursday. Meanwhile, upper level ridging keeps conditions warm throughout the West and central U.S. the entire period, with a handful of both record highs and warm overnight lows possible. The greatest anomalies should be across the northern-central Plains where daytime highs could be 20 to 30 degrees above normal. As the flow flattens during the late period, above normal temperatures should also begin expanding into the Midwest and South. 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