Extended Forecast Discussion NWS Weather Prediction Center College Park MD 103 AM EDT Wed Aug 08 2018 Valid 12Z Sat Aug 11 2018 - 12Z Wed Aug 15 2018 ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles show reasonably good clustering through much of period with the upper flow across the CONUS as an upper low moves through southern British Columbia Saturday and another upper low sinks southward toward the Ohio Valley. As the Canadian trough approaches the Great Lakes it will kick out the Midwest system into New England as both race toward the northwestern Atlantic. Utilized a blend of mostly the 12Z/18Z GFS and 12Z ECMWF as they paired well with their ensemble means. However, a rather significant change was made for the forecast next Tuesday in the Northwest as upper ridging is now favored just off the coast with the trough much farther east like the ECMWF's solution 24 hrs ago (which was unlike nearly all ensembles). To the east, the GFS has been more consistent with the upper low in the Midwest, so a blend sufficed on both accounts given lingering/typical uncertainty. ...Weather Highlights and Hazards... Near or above record high temperatures over the interior Northwest/northern Rockies/High Plains Saturday will trend cooler Sunday/Monday as the upper trough and surface cold front push through. Southwestward extension of the upper low over Ohio on Sunday/Monday will help fuel organized rain/storms across Texas westward to the Four Corners region, especially NM/CO just to the east of the upper high. Guidance continues to waver on placement but several inches of rain are likely over central Texas during the Sat-Wed time period. Wavy front will linger across the U.S. southern tier during the period which will help spark afternoon showers/storms and locally heavy rainfall in the warm/humid airmass. As the upper trough (eventual closed low) digs/sharpens over the east-central U.S., more widespread precipitation may be favored to its east along the Appalachians to the I-95 corridor. This could develop into a more widespread heavy rainfall threat considering slow system translation and amplitude with a southerly fetch of ample moisture over several days. Closed lows in the summer can be underestimated in the medium range in terms of precipitation intensity/amounts. Finally, approaching frontal boundary over the High Plains/Upper Midwest next week will act as another focus for shower/storms just to its south as the leftover weakness aloft gets absorbed into the next trough. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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