Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1131 AM EDT Thu Aug 23 2018 Valid 12Z Sun Aug 26 2018 - 12Z Thu Aug 30 2018 ...Pattern Overview... A strong upper high south of the Gulf of Alaska on Sunday will drift slowly westward next week. To its east, troughing will linger and be reinforced over the Pacific Northwest while an upper high in the Southeast drifts eastward. This will act to keep any frontal boundaries near the US/Canadian border (Great Lakes to the Northeast) as surface low pressure lifts through the Upper Midwest Mon/Tue. Tail end of that system will meander across the central Plains Tue-Thu. ...Guidance Evaluation/Preferences... 00Z/06Z models/ensembles showed more spread than normal in the Northwest Sun-Mon with the lead trough but better than average agreement in the east with the ridging. The ECMWF and its ensembles were quicker to lift the trough eastward compared to the other 00Z guidance and its previous (12Z/22) runs in response weaker mid-level vorticity on the east side of the upper high. Though its previous runs seemed too aggressive/overdone, the 00Z runs of the ECMWF/EC-ENS may be too weak/quick as the absolute east-west ensemble spread has not changed much between the past two cycles. 00Z Canadian was the slowest/deepest (with a closed low) and was not preferred past Monday. Preferred to yield about 25% toward the ECMWF-led quicker solutions and 75% with continuity/GFS/GEFS given uncertainty way upstream with Tropical Storms Cimaron/Soulik in the W Pac as they lift into the westerlies. For next Wed-Thu, positively tilted trough in the west will mostly linger as the upper high in the east tries to push toward New England (at least maintaining higher than average heights). Another trough may try to push into the Pac NW again next Thursday through there has been a large amount of E-W spread in its axis, again related to upstream W Pac flow atop the upper high. Increased the ensemble weighting in turn to account for the uncertainty. Fracasso ...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