Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Mon Aug 06 2018 Valid 12Z Thu Aug 09 2018 - 12Z Mon Aug 13 2018 ...Record heat for the interior Northwest Thursday/Friday... ...Pattern Overview and Guidance/Predictability Assessment... Ejection of the eastern Pacific upper low and associated trough energy across the extreme northwestern U.S. and southern Canada will gradually suppress upper ridging over the West and push the northern part of the initial western ridge into the Plains and upper MS Valley. Toward the start of next week additional energy may feed into the mean trough becoming established over western Canada/northwestern U.S., in response to upstream amplification as the extratropical evolution of typhoon Shanshan brings a deep low into the Aleutians/Bering Sea and ridging builds over the northeastern Pacific. Meanwhile expect upper troughing to sharpen over the east-central U.S. with an embedded upper low possibly closing off during the weekend. Reviewing latest guidance, the current cycle has furthered the trend toward somewhat faster ejection of the eastern Pacific upper low/trough albeit with less of an adjustment than seen 12-24 hours ago. This is a sensitive part of the forecast given expected interaction with northeastern Pacific energy, and the full ensemble envelope is sufficiently wide to suggest potential for more adjustments by the models. Latest model/mean consensus that digs a bit more energy into the Pacific Northwest/western Canada trough by the end of the period seems reasonable given the amplification expected to occur upstream. Over the eastern U.S., recent model runs and latest ensemble mean trends seem to signal a greater than even chance that an upper low will close off within the sharpening trough. However there has been variability on where the upper low will close off and the 00Z ECMWF does not close off a low until eastern Canada energy drops into the mean trough after the end of the period. For days 3-5 Thu-Sat a blend of latest operational guidance (00Z-06Z GFS and 00Z ECMWF/UKMET/CMC) represented consensus while accounting for existing spread of timing/details. Days 6-7 Sun-Mon introduced 06Z GEFS/00Z ECMWF mean guidance with model and mean components given equal weight by late in the forecast. ...Weather Highlights and Hazards... Expect near or above record high temperatures over much of the interior Northwest Thursday/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. 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. 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 low. Rausch/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