Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1252 AM EDT Tue Aug 07 2018 Valid 12Z Fri Aug 10 2018 - 12Z Tue Aug 14 2018 ...Record heat for the interior Northwest Friday... ...Pattern Overview and Guidance/Predictability Assessment... Latest 12Z/18Z models and ensembles showed good clustering through the 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 toward the Ohio Valley Sun/Mon next week. By next Tuesday, Atlantic ridging will push westward which should bump the upper low to the northeast through the eastern Great Lakes as troughing settles into the Pacific Northwest. Used a deterministic blend to start with less emphasis on the Canadian (too slow with Hurricane John) then the ECMWF (too quick to lift the eastern upper low northward and too quick with western Canadian troughing by Monday compared to the ensembles). The recent GFS runs were maintained and showed good continuity. A couple of surface boundaries in the east will eventually coalesce around the upper low by next week. The upper low off the west coast will drag a cold front eastward across the High Plains. ...Weather Highlights and Hazards... Expect near or above record high temperatures over much of the interior Northwest Friday 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. Heavier convective precipitation is likely in the 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. Clouds/rainfall should promote below normal high temperatures over this region for most of the period. Precipitation will accompany and precede a frontal boundary as it pushes through the Great Lakes and into the Northeast via afternoon storms. Another 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. Distribution/intensity of rainfall will be sensitive to the exact evolution of the upper trough and possible closed low. Humid airmass will remain in place to its east up toward the Northeast. 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