Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Nov 28 2021 Valid 12Z Wed Dec 01 2021 - 12Z Sun Dec 05 2021 ...Overview... The overall pattern will be featured by an upper-level ridge from the eastern Pacific to the Intermountain West, and quasi-zonal flow to low amplitude troughing from the Plains to the East Coast with multiple shortwave perturbations pivoting around a synoptic scale trough over southeast Canada. This will include an organized low pressure system tracking across southern Canada through Friday with a trailing front followed by a weaker low this weekend, leading to rounds of light to moderate precipitation across the north-central and northeastern U.S. By the weekend, moist inflow is expected to return to the south-central U.S., bringing increased rain chances as the moisture focuses along a frontal boundary. ...Guidance/Predictability Assessment... The 00/06Z model cycle begins the medium range period Wednesday with fairly good agreement with the large scale pattern: ridging through western parts of the U.S. while shortwaves (with varying intensities depending on the model) pass through mild troughing in central/eastern parts of the country. A particularly strong shortwave will push a surface low through southern Canada Wednesday-Thursday with reasonably good agreement in position. GFS runs were the strongest with a shortwave tracking quickly through the Southeast late Wednesday into Thursday, which was not particularly favored. Magnitude and positioning of shortwaves continue to be the biggest forecast challenge Friday into the weekend as well, causing some uncertainties to arise even within the broader flow by the weekend. In general the flow by the weekend has trended less amplified compared to model guidance, with the exception of a southern stream upper low trending a little stronger across northern Mexico by Sunday, and at this point it does not appear there are clear outliers with the overall pattern. Thus the WPC forecast was based initially on a blend of deterministic guidance of the 00Z/06Z GFS and 00Z ECMWF, UKMET, and CMC, phasing toward a blend of the models and GEFS and EC ensemble means by the end of the period, along with some continuity. ...Weather/Hazards Highlights... Weather should be relatively quiet in terms of rainfall or snow related hazards with not much in the way of heavy precipitation currently expected. Generally moderate precipitation lingering in the Pacific Northwest, particularly the Olympic Peninsula and Northern Cascades, should abate for the latter part of the week as a surface high builds over the Pacific Northwest and British Columbia, before precipitation chances increase again for the weekend. Rounds of relatively light precipitation are forecast for the Midwest, Great Lakes, and Northeast with a progressive storm track over the area. Amounts should be most substantial in favored lake effect and possibly upslope areas, particularly downwind of Lake Ontario into the Adirondacks. Then moisture is set to increase across the Southern Plains and Lower Mississippi Valley late Friday and into the weekend, which could lead to rain becoming more widespread there and northeastward depending on possible low pressure and frontal system development. Temperatures are expected to be well above normal for this time of year across the western U.S. and the Plains through the end of the week owing to the ridge aloft, and closer to normal for the East Coast on Wednesday followed by a warming trend for Thursday and Friday. Both highs and lows are forecast to be 15 to 25+ degrees above average for northern and central portions of the High Plains and expanding into the Midwest for Wednesday and Thursday. A few locations may approach daily maximum and minimum temperature records with these mild temperatures. By the weekend, temperatures are currently forecast to moderate somewhat, but this is subject to change given forecast uncertainties. Tate/Hamrick 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