Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 AM EDT Sat Sep 18 2021 Valid 12Z Tue Sep 21 2021 - 12Z Sat Sep 25 2021 ...Weather Pattern Overview... The strong storm system that is currently affecting the Pacific Northwest is expected to be over the general vicinity of the northern Great Lakes by Tuesday morning, accompanied by a strong cold front that should steadily progress across the eastern third of the nation. The large surface high and upper ridge that will be across the eastern U.S. early in the week will eventually give way to the approaching trough and cold front, which will herald a pattern change compared to the recent extension of summer-like heat and humidity. In addition, a second storm system is forecast to reach the northwestern U.S. by Wednesday night and into Thursday as the storm track across the northeast Pacific remains active next week, along with a possible closed low over the Desert Southwest by the end of next week. ...Overview and Guidance Evaluation/Preferences... The latest deterministic guidance is initially in decent agreement on Tuesday across most of the continental U.S. before rather significant differences emerge as early as Wednesday night and below average confidence exists for the second half of the forecast period. The long wave trough crossing the Midwest may evolve into a slower moving closed low over the Ohio Valley based on the non-NCEP guidance, and thus a slower cold front progression compared to earlier forecasts, with the UKMET the slowest solution here. The GFS and its ensemble mean bring the cold front and shortwave through faster before evolving into a closed low over the Northeast states by Thursday night. This has implications for the next shortwave/front crossing the Rockies and northern Plains by the end of the week, with the GFS progressive and more amplified. Substantial uncertainties also exist with a potential weak closed low near California as suggested by the 12Z ECMWF, whereas the bulk of the guidance is more suggestive of a broad upper ridge in this region. The WPC fronts/pressures forecast was primarily based on a nearly equal blend of the CMC/GFS/ECMWF through Wednesday, and then slightly more weighting on the ECWMF while still maintaining some of the GFS and CMC through Thursday. More of the ensemble means were incorporated for Friday and Saturday, and some previous WPC continuity was maintained through day 6 owing to the model variations noted above. Noteworthy changes in the forecast are possible in future updates for Thursday and beyond. ...Weather and Potential Hazards... Widespread showers and scattered thunderstorms are expected to develop along and ahead of the cold front from the Tennessee River Valley to the Great Lakes midweek, with some locations getting on the order of 1 to 2 inches of rainfall with locally higher totals possible. This unsettled weather is expected to reach the Mid-Atlantic and Northeast by Thursday with locally heavy rainfall possible. Some strong to severe storms will also be possible where favorable parameters of wind shear and instability exist, although location specifics are uncertain. There may also be enhanced fire weather concerns for portions of the West, depending on the magnitude of the pressure gradient and resulting winds in the wake of the cold front. In terms of temperatures, it will be pleasantly cool behind the cold front across the Plains and Midwest for the middle of the week, and the fall-like conditions 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, although no major heat waves are currently anticipated. 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