Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1120 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 now show better than normal clustering through the period for the CONUS, bolstering forecast confidence. The pattern offers an eastern Pacific upper low that moves through southern British Columbia Saturday as a lead upper low sinks slowly southward into the Ohio Valley and closes off. Subsequent Canadian system approach toward the Great Lakes early next week kicks the lead Midwest system into New England. The WPC medium range product suite was mainly derived from the 00/06 UTC GFS and 00 UTC ECMWF along with their well paired respective ensemble means and WPC continuity. ...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. A separated southwestward extension from the OH Valley upper trough by 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 in a region of favorable upper diffluence. Guidance continues to waver on placement but several inches of rain are likely over central Texas during the Sat-Wed time period. A 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 short and medium range in terms of precipitation intensity/amounts and longevity. Upstream, an approaching frontal boundary over the High Plains/Upper Midwest/Great Lakes 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/Schichtel 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