Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Sun Sep 19 2021 Valid 12Z Wed Sep 22 2021 - 12Z Sun Sep 26 2021 ...Heavy Rainfall Threat from the Great Lakes/Ohio Valley to the Northeast states... ...Weather Pattern Overview... The longwave trough that will be crossing the central U.S. is forecast to evolve into a closed upper low over the Ohio Valley and Great Lakes region by Thursday, slowly lifting to the northeast by the end of the week as the cold front exits the East Coast. Another trough and surface cold front will be tracking from the Pacific Northwest to the northern Plains by Friday, although not as strong as the preceding system. In its wake will likely be a developing upper ridge centered over the Intermountain West for next weekend, along with a weak upper low near California that may bring some moisture northward across portions of the Desert Southwest. ...Overview and Guidance Evaluation/Preferences... The latest deterministic guidance is in better agreement through Friday across most of the continental U.S. compared to this time yesterday. The long wave trough crossing the Midwest is forecast to evolve into a slower moving closed upper low over the Ohio Valley, and thus a slightly slower cold front progression compared to earlier forecasts, with the ECMWF the slowest solution here. By Friday night, the GFS is a bit faster with the next shortwave trough crossing the Northern Plains and Upper Midwest, but still within the ensemble spread. Confidence decreases more significantly by next Sunday with greater timing differences noted in northern stream flow across the northeast Pacific and near the Canadian border, and also regarding the extent of the upper ridge over the Intermountain West. The WPC fronts/pressures forecast was primarily based on a nearly equal blend of the latest available deterministic guidance through Thursday, with slightly more weighting on the 12Z UKMET given its middle ground solution with the closed low over the Ohio Valley that is also close to the ICON and JMA models. More of the ensemble means were incorporated for the second half of the forecast period owing to increased model spread across the northern tier. ...Weather and Potential Hazards... A corridor of moderate to heavy rainfall is becoming increasingly likely from the Ohio Valley to the Northeast U.S. for the Wednesday through early Friday time period. The potential exists for widespread 1-3 inch rainfall totals, with isolated amounts up to 4 inches possible over a two day period. Much of this rainfall will be associated with the upper level flow that develops, and some instances of flooding may become realized if higher rainfall rates repeatedly affect the same areas and for those regions with saturated soils. There may be some strong thunderstorms as well, but the overall severe potential appears limited for now. In terms of temperatures, it will be pleasantly cool behind the cold front across the Midwest and the Ohio Valley for the middle of the week, and the cold front should make it to the East Coast by Friday along with improving humidity levels. A warming trend is likely to commence across much of the western U.S. by next weekend in response to a building upper level ridge, with highs expected to be on the order of 10 to 20 degrees above normal across the western High Plains next Sunday. Meanwhile, temperatures should be close to seasonal averages for the eastern half of the nation for this same time period. 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