Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Wed Aug 07 2019 Valid 12Z Sat Aug 10 2019 - 12Z Wed Aug 14 2019 ...Overview and Guidance/Predictability Assessment... A zone of relatively progressive flow across the CONUS northern tier will be squeeze between blocked flow at higher latitudes in Canada, and a persistent upper ridge anchored across the south central U.S. The GFS differed from the consensus of model guidance even on day 3 (Sat) in this area of progressive flow with respect to a shortwave crossing the Canadian prairies and skirting the Great Lakes, showing a solution nearly out of phase with the 12Z ECMWF/CMC/UKMET as well as the 12Z ECENS/18Z GEFS ensemble means (although the 00Z run of the CMC did join the GFS). Model consensus was relatively good with an upper low reaching the U.S. West Coast on Sat. Solutions showed relatively small timing/amplitude differences Sun-Mon as the system opens into a broader upper-level wave and crosses from the northern Great Basin to the Northern Plains. Models show this feature continuing to lose amplitude and broaden out as it moves east Tue-Wed across the Great Lakes, moving a cold front east into the Ohio Valley. Finally, models seem to have settled more on a less progressive trough off the U.S./Canada west coast by the early to middle portion of next week - a solution that was previously preferred due to an upstream blocking ridge across Alaska. Given these considerations, the WPC forecast was initially based heavily on a blend of the 12Z ECMWF and CMC during days 3-5 (Sat-Mon). A gradual increase in weighting of ECENS/GEFS ensemble means was shown through time, with the majority of the forecast comprised of ensemble means by days 6-7 (Tue-Wed). ...Weather Highlights/Threats... Showers and thunderstorms will focus along a cold front moving from the northern Rockies into the Northern/Central Plains Sat-Sun, with areas of heavy rainfall possible. Convection with locally heavy rains will continue to focus from the Central Plains east to the Mid-Mississippi and Ohio Valleys early next week as the frontal system moves east. Meanwhile, the weakening remains of another frontal boundary will focus scattered showers and thunderstorms across the Southeast through the forecast period. Below average temperatures will accompany the upper low/trough into the northwestern U.S., where high temperatures Sat-Sun are forecast to be 5 to 15 deg below average. These temperatures anomalies are forecast to spread east into the Northern Plains by early next week. Meanwhile, hot conditions will persist across the Southern Plains beneath the upper-level ridge, with high temperatures forecast to be 5 to 10 deg above average into next week. Ryan 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