Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM 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 should 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 timing and magnitude differences, which would affect a surface low deepening off the Northeast coast. The 18z yesterday (and todays 00z) GFS continues to be strongest/deepest with the upper low while the 12z/yesterday ECMWF intially keeps a surface low farther south along the Mid-Atlantic coast before jumping north and more in line with the other solutions. The ensemble means seem to offer a solution clustered closer to the GFS/CMC/UKMET which is what WPC favored tonight. Regardless, the placement of this surface low will have impacts for the Northeast including how much precipitation falls, so this will need to be monitored. Out West, a weak shortwave into the West Coast around Monday next week should propagate eastward into the Central Plains and eventually the East by the middle to latter part of next week. The last couple of model runs indicate there's plenty of uncertainty remaining with this system, especially the latter half of the period mainly timing and intensity and how it interacts with the Eastern trough already in place. A blend towards the ensemble means later in the period seemed to mitigate these differences well until these details can be ironed out in future runs. ...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. Westerly flow across the Great Lakes should continue to support lake effect snowfall with some light to moderate accumulations possible Sunday into Monday. Models show the heaviest rainfall associated with the early period Northeast surface low should mainly remain offshore, but wrap around light snow or coastal rain may be possible. An upper low camped out west of Baja California may help funnel moisture towards the Gulf Coast region, but associated precipitation should be light and scattered in nature. After Monday, conditions across the CONUS should begin to dry out for most regions as the flow becomes less amplified. The exception to this may be across the Pacific Northwest where troughing in the northeast Pacific should keep a consistent focus of moisture into the region. 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. 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