Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 AM EDT Fri Sep 17 2021 Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021 ...Weather Pattern Overview... The strong storm system that is currently approaching the Pacific Northwest is expected to be over the central and northern Plains by Monday morning, accompanied by a rather strong cold front that should steadily progress across the central and then eastern U.S. The large surface high and upper ridge across the eastern U.S. will eventually give way to the approaching trough and cold front, which will herald a pattern change for much of the eastern half of the nation. In addition, a second storm system is forecast to reach the northwestern U.S. by Wednesday and into Thursday as the storm track across the northeast Pacific remains active next week. ...Overview and Guidance Evaluation/Preferences... The deterministic guidance is in above average agreement for the beginning of next week, with the 18/00Z GFS beginning to show a faster solution by Tuesday night with the trough/cold front crossing the Great Lakes and Midwest region. This is even more apparent with the second trough crossing the northern Rockies and northern Plains by Thursday, and also ahead of its own ensemble mean. By next Friday, model spread becomes substantial across much of the northern tier states and southern Canada, with much of this owing to the eventual evolution of a possible closed upper low over southern Manitoba as the second Pacific trough moves inland. The WPC front/pressures forecast was primarily based on a nearly equal blend of the 12Z deterministic guidance through early Wednesday, and then primarily the 12Z GFS/ECMWF and their respective ensemble means through Thursday and Friday. The 12Z run of the GFS was substituted for the 18Z run since the latter was considerably faster with the cold front across the eastern U.S. compared to the model consensus, whereas the former is more in line with the CMC/UKMET/ECMWF. ...Weather and Potential Hazards... Widespread showers and scattered thunderstorms are expected to develop along and ahead of the cold front from the Midwest to the Northeast and Mid-Atlantic states, with some locations getting on the order of 1 to 2 inches of rainfall with locally higher totals possible. Some strong to severe storms will also be possible where favorable parameters of wind shear and instability exist, although location specifics are uncertain. Winter weather will also be likely across the higher terrain of northern Wyoming and western Montana with accumulating snow likely on Monday, followed by a drier weather pattern by Tuesday and beyond. 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. It will still feel like summer across much of the southern Plains and extending northward to the Great Lakes and the Ohio Valley to begin the week, and more like late October across much of the Rockies and Intermountain West with highs running 10 to 20 degrees below average. These cooler temperatures are expected to reach the Midwest by midweek and finally to the East Coast by Thursday and Friday, along with much more pleasant humidity levels. 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