Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Sat Jul 28 2018 Valid 12Z Tue Jul 31 2018 - 12Z Sat Aug 04 2018 ...Hot temperatures over the West moderating... ...Heavy rainfall threat over portions of the East... ...Pattern Overview and 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. ...Guidance/Uncertainty Assessment... The 18Z GFS/12Z ECMWF and their ensemble means (plus the 12Z UKMET days 3-5 Tue-Thu) agreed fairly well in principle through the period. The operational model cluster represented consensus through mid-period, with ensemble guidance reaching half of the total forecast weight by day 7 Sat. CMC/CMC mean runs continue to be weaker with the western Atlantic ridge, allowing eastern U.S. height falls aloft/surface low pressure to extend farther east than consensus. Interaction between amplifying southern Canada energy and the MS Valley trough around midweek still leads to some detail uncertainty in the forecast over eastern North America. There are still significant question marks over specifics of ejecting upper low energy that is near Vancouver Island as of Wed. However as was the case yesterday there is still decent model/ensemble agreement for the general area of surface low pressure emerging over the northern Plains/southern Canada. The progressive nature of mean flow late in the period may make it difficult to resolve finer details until the short range time frame. Another influence on the forecast that may come into play late in the period is the amplification expected across the Pacific. Thus far there have been some significant differences on the placement/amplitude of strong North Pacific/Aleutians ridging and resulting northeastern Pacific trough, with GFS runs tending to be somewhat amplified/shifted a little west versus consensus for the overall pattern. Teleconnections relative to the Aleutians positive height anomaly center depicted in the D+8 multi-day means seem to favor leaning closer to the ECMWF/ECMWF mean over the northeastern Pacific at that time. 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