Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 AM EDT Thu Aug 23 2018 Valid 12Z Sun Aug 26 2018 - 12Z Thu Aug 30 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Confidence is high that a strong upper-level ridge will be in place south of Alaska at the start of the medium range. Downstream effects of this ridge will be a mean trough across the northwestern U.S., and expansive upper ridging from the southern plains to the Mid-Atlantic. An active flow regime will persist across the northern tier as a number of shortwaves eject from the western mean trough cross the northern plains/Upper Midwest to the Great Lakes and New England, generating multiple waves along a surface frontal boundary. As in recent days, solutions with a more amplified trough across the western states were preferred given the highly amplified flow expected to evolve across the North Pacific. Initially, these more amplified solutions were GFS/CMC, with the ECMWF joining the consensus later in the period. As smaller scale energy ejects eastward in the northern stream flow throughout the period, models show expected timing/amplitude differences. The most significant piece of energy to move eastward across the northern plains looks to occur Tue-Wed, generating a wave along the surface frontal boundary from the northern/central plains to the Upper Midwest. Given these considerations, the forecast for days 3-5 was based on a multi-model deterministic blend, with more weight initially placed toward the GFS/CMC. By mid to late next week, additional shortwave energy looks to enter the western mean trough with amplification likely to occur. Models begin to diverge by this time period as to the timing and exact character of this feature, with some solutions showing a deep closed upper low before the system reaches the Northwest, and others more quickly moving the energy into the mean trough with amplification across the Great Basin. Given the increase in spread, a trend toward ensemble means (ECENS and NAEFS) was preferred for days 6-7. ...Weather Highlights and Hazards... The most active weather in terms of precipitation will be across the northern tier during the medium range given the trough/shortwave energy and frontal boundary described above. Models show the potential for multiple areas of potentially heavy convection across multiple days from the northern Rockies to the Great Lakes. By next Tue-Wed, some convection may spread as far east as the Northeast. The consensus of model/ensemble solutions suggest that the greatest potential for heavy rainfall will exist across portions of the Upper Midwest and Great Lakes, as the frontal boundary will be in the vicinity through most of the forecast period. Farther south, scattered afternoon/evening showers and thunderstorms will persist across the Southeast as a stationary surface frontal boundary slowly decays. Monsoonal convection in the Four Corners region should gradually reduce in coverage/intensity through the medium range as the low/mid-level flow becomes less favorable for transport of deep moisture into the region. Expect below average temperatures across much of the northwestern U.S. given the persistent upper trough overhead. High temperatures of 5 to 15 deg F below average are expected into early-mid next week. Farther east, the expanding upper ridge will support increasing temperatures, with highs 5 to 10 deg F above average across much of the central and eastern U.S. through the period. Ryan 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