Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1210 PM EDT Sat Jul 28 2018 Valid 12Z Tue Jul 31 2018 - 12Z Sat Aug 04 2018 ...Heat wave moderating over the West... ...Heavy rainfall threat over portions of the East... ...Pattern Overview and Guidance/Uncertainty Assessment... Model guidance this morning showed decent agreement on the synoptic pattern evolution across the U.S. through next Saturday. Key features include a Bermuda High remaining firmly established from the western Atlantic to the East Coast while a positively-tilted trough nudges its way eastward closer to the Appalachians. Meanwhile, a cool Canadian high pressure system is in the offing for the northern Plains and Great Lakes midweek next week before a low pressure system attempts to organize across the Pacific Northwest late next week reaching the northern Plains by next weekend. The 00Z ECMWF has switched to a warm solution over the northern Plains midweek with a much weaker high pressure system coming down from Canada than what the GFS indicates. Therefore, less of the ECMWF has been used for the max/min temperatures over the northern Plains for that time frame. Otherwise, a blend of the 00Z ECMWF and the 06Z GFS should provide a good starting point. For Days 6 and 7, the 06Z GFS and GEFS show solutions that agree better with the 00Z ECMWF/EC mean than what the 00Z GFS/GEFS show. Therefore, a general compromise between the 00Z ECMWF and the 06Z GFS is used to derive the WPC grid package, trending closer to their ensemble means for Days 6 and 7. Kong ...Weather Highlights/Threats... Through a combination of some energy ejecting northeastward and the remainder weakening in place, expect the amplified trough aloft over or just east of the MS Valley as of Tue-Wed to become more diffuse thereafter. The eventual weakening trend for both this trough and the initial western U.S. ridge will lead to a flow evolution that more closely resembles a mid-late summer pattern by Fri-Sat with low amplitude westerlies across the Northern Tier. Strong ridging over the western Atlantic will be a more persistent feature. Over the East, the first half of the period will feature MS Valley low pressure tracking northeastward and associated leading warm/trailing cold fronts providing well-defined focus for convection and heavy rainfall potential. The cold front will have difficulty reaching any farther than near the East Coast due to the strong deep-layer ridge over the western Atlantic. Moist Gulf/Atlantic flow around the Bermuda high will likely interact with the stalling front and maintain the possibility of significant rainfall over parts of the Appalachians/Southeast/Mid-Atlantic. The front will likely dissipate by Fri-Sat but some diurnally favored convection should continue with more of a terrain/localized boundary focus. The best signal for highest 5-day totals exists over the southern Appalachians/Southeast, while other relative maxima are possible over the Ohio Valley/eastern Great Lakes and parts of the Mid-Atlantic. Flattening of the western U.S. ridge will be aided by the eastward progression of energy within/around an upper low still forecast to be near northern Vancouver Island as of early Wed. This upper support will bring a general area of low pressure with one or more centers into the northern Plains/southern Canada by the latter half of the period--led by a warm front representing the northward return of the cold front that initially drops into the Northern Tier Tue-Wed. The initial cold front will bring some precipitation to northern areas and then the late-period system should lead to one or more areas of convection--some possibly heavy--within an area between the northern Plains and western Great Lakes. Meanwhile expect diurnally favored convection of varying intensity to persist over the Four Corners states, though with Utah tending to see lighter/more scattered activity than the rest of the region. The pattern change aloft over the Northwest will bring a pronounced cooling trend to that region. Some locations in the interior may see highs 10-15F above normal Tue but then expect modestly below normal highs by Thu-Sat. From the Great Basin southward min temps will likely remain well above normal through the period while anomalies for highs should be less extreme. Most of the central U.S. will see well below normal readings especially Tue-Wed with lesser cool anomalies for highs extending into the Southeast for most of the period given persistent clouds/rainfall. On the other hand moist flow will keep min temps well above normal over the Mid-Atlantic/Northeast. After the cool start to the period, the Northern Tier will see temperatures rebound late in the week as southerly flow develops ahead of northern Plains low pressure. Rausch 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