Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Sat Sep 18 2021 Valid 12Z Tue Sep 21 2021 - 12Z Sat Sep 25 2021 ...Heavy Rainfall Threat from the Great Lakes/Ohio Valley through the Mid-Atlantic/Northeast... ...Guidance Evaluation/Preferences and Weather/Threats Highlights... Medium range model forecast spread has decreased Tue into Wed, but uncertainty grows through later next week. The WPC medium range product suite was primarily derived from a blend of the 00 UTC ECMWF with the 06 UTC GFS/GEFS and the 13 UTC National Blend of Models. The last few ECMWF ensemble runs were not available at WPC due to computer issues. There has been a significant recent trend in most guidance toward more robust flow amplification through medium range time scales and this WPC blend adjusts product continuity in that direction. The 12 UTC GFS has bucked this trend, offering limited outlier uncertainty, but the rest of newer 12 UTC guidance remains in line to bolster forecast confidence. In this scenario, potent system energy set to dig through the Midwest may prove to be a gradually progressive heavy rainfall producer and local runoff threat from the Great Lakes/OH Valley to the Mid-Atlantic Northeast Tue-Thu, perhaps lingering into Fri over the New Eng depending on uncertain upper system ejection by approaching upstream northern stream energies. The threat is expected to be fueled by pooling moisture/instability as guidance now develops a supporting closed upper low and slows progression. The system will be accompanied by a strong and focusing cold front that will progress across the central and eastern states that would also provide some focus for more modest rainfall down across the South/Southeast. The lead 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 front, which will herald a pattern change compared to the recent extension of summer-like heat and humidity. Steady and amplified upstream upper trough progression through western Canada now noted in guidance Wed/Thu would also be primed as a kicker to dig into the north-central U.S./Midwest later next week along with another associated frontal system, but with much less precipitation potential with depleted moisture availability in the wake of the main lead system. Best potential may over vicinity of the Great Lakes under the meat of the trough. Meanwhile, weaker eastern Pacific upper trough energy set to reach the Northwest midweek is trending to now dig down the West Coast and eastern Pacific to offer minimal additional precipitation potential for the region. Schichtel 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