Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Tue Aug 07 2018 Valid 12Z Fri Aug 10 2018 - 12Z Tue Aug 14 2018 ...Record heat lingers for the interior Northwest to Northern High Plains and Southern California into Saturday... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles show reasonably good clustering through much of period with the upper flow across the CONUS. This will take an upper low initially just off the Pacific Northwest coast northeastward into southern British Columbia then into south central Canada as another upper low sinks southward to the OH Valley early next week. Western Pacific Typhoon Shanshan is forecast to become extratropical and track to Alaska later this week. Prefer a solution that maintains a deepened surface low and lead amplifying flow aloft. The 00 UTC GFS/Canadian and experimental FV3 offer the most downstream amplitude and the 00 UTC GEFS/NAEFS offer the most ensemble mean amplitude. Discounted less amplified 00 UTC ECMWF/ECMWF ensemble solutions. ...Weather Highlights and Hazards... Expect near or above record high temperatures from over the interior Northwest into the northern Rockies/High Plains Friday into Saturday ahead of the eastern Pacific upper low with 100s widespread east of the Cascades and through the Snake Valley in Idaho as well as the Missouri River Valley in Montana. Light precipitation will be confined to coastal/northern/higher elevation areas of Washington as the best upper support travels through Canada, though that may dip south again in eastern Montana to the Dakotas as some moisture flows northward east of the Rockies into a lead front to focus convection. Expect heavy convective precipitation in deeper moisture on the eastern side of the western U.S. upper high, from NM/CO/AZ eastward to the southern Plains. It is likely that some locations will see several inches of rain over a few days which may lead to localized flooding. Ensembles have been targeting Texas for a few runs though the favored location may shift from day to day. Precipitation will accompany and precede frontal boundaries over the OH Valley and Northeast. A lead 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 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. Distribution/intensity of rainfall will be sensitive to the exact evolution of the upper trough and possible closed low. The humid airmass will remain in place to its east. 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