Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri Aug 28 2020 Valid 12Z Mon Aug 31 2020 - 12Z Fri Sep 04 2020 ...Threat of Heavy Rain early to middle of next week across south-central Plains to mid-Mississippi Valley... ...Overview and Guidance/Predictability Assessment... Over the course of the medium range period (Mon 8/31-Fri 9/4), model guidance is in fairly good agreement on a shortwave trough moving relatively slowly across the central Plains while the faster northern stream sends reinforcing waves of cooler air from Canada into and across the northern tier and down toward the central Plains. The deterministic solutions are showing a good signal for the upper level energy to fracture off into a closed low over south-central Plains by about next Wednesday. The 00Z Canadian model appears to be the fast outlier while the last two runs of the GFS agree quite well with the 00Z ECMWF regarding that upper trough. The 00Z EC mean was also most agreeable with the 00Z and 06Z GEFS means compared with the Canadian mean as the models tend to push the shortwave toward the eastern U.S. by the end of next week. Therefore, the morning WPC prognostic charts were based on a blend of the 00Z ECMWF and the 06Z GFS together with an increasing use of their ensemble means toward the end of the medium range period. ...Weather/Hazard Highlights... Models continue to show a good signal for precipitation to be focused across portions of the south-central Plains toward mid-Mississippi River Valley from Monday through Wednesday near/north of a wavy stationary boundary as the upper-level shortwave approaches. Episodes of heavy showers and thunderstorms will likely result in heavy rainfall in these areas. By the middle to later portions of the week the front washes out as the upper level energy moves eastward and any threat for heavy rainfall may shift toward the Ohio Valley and Great Lakes. Early autumn like temperatures are expected across much of the northern Rockies and central to northern Plains early in the medium range period where daily highs in the 60s to 70s will be from 5-15F below normal. Morning lows in the 30s to 40s (20s in the terrain areas of the Rockies) will be up to 10F below normal where northwestern Wyoming will likely experience the first freeze of the season Monday morning. Elsewhere, temperature anomalies are generally expected to be 5F or less. By the end of next week, a reinforcing shot of cool air will appear to push into the northern part of the country. Lastly, models are also trending toward a stronger upper high over the Southwest by the end of next week, thereby increasing the chance of heat to build across the Desert Southwest. Kong/Taylor Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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