Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Thu Aug 02 2018 Valid 12Z Sun Aug 05 2018 - 12Z Thu Aug 09 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... Latest guidance is reinforcing trends that emerged 24-36 hours ago, toward the idea of an upper low dropping into the eastern Pacific becoming cut off from the westerlies for most of the medium range time frame--keeping the feature well offshore the West Coast while a strong ridge builds over the western U.S. and into Canada. In response to this ridge expect mean troughing to develop over eastern North America, ultimately leading to weakening/suppression of the upper ridge that should be in the process of building into the Southeast during the weekend. Based on the current consensus including the slower-trending 00Z CMC the eastern Pacific upper low may not begin to eject northeastward until around day 7 Thu, so leading height falls may not reach the Pacific Northwest until then. Operational runs generally led the ensembles as the rapid adjustment in guidance occurred over the past couple days, meriting operational model emphasis early-mid period and at least half weight of the operational runs late. Over the eastern 2/3 of the continent, various shortwave uncertainties will be the primary forecast issue. Low amplitude flow initially over the Northern Tier will likely contain at least a couple trackable shortwaves or shortwave clusters--one over the northern Plains/upper MS Valley as of early Sun and a second progressing from the Northwest into the northern Plains. This trailing feature and then another shortwave possibly reaching into Canada during the first half of next week should amplify into the expected eastern mean trough aloft. Depth and timing of separate shortwaves vary among model runs, ensemble members, and between consecutive cycles so confidence is a lot lower for the component features within the more highly predictable overall trough pattern. This favors a model/mean approach especially by the latter half of the period. Based on guidance through the 12Z/18Z cycles, the updated forecast used input from the 12Z-18Z GFS/12Z ECMWF and to a lesser extent the 12Z CMC/UKMET into day 5 Tue. The forecast introduced the 18Z GEFS/12Z ECMWF means late Tue with ensemble weight reaching 50 percent of the total by day 7 Thu. ...Weather Highlights/Threats... With the latest cycle of guidance either consistent with or furthering trends toward keeping the eastern Pacific upper low well offshore through the period while upper ridging builds over the West, confidence is increasing that most of the western U.S. should see a warming trend to above normal temperatures by Tue-Thu. Dry conditions will likely prevail over most areas as well, aside from diurnally favored convection at times over parts of the Four Corners states. Before this warm-up locations from the central West Coast into the northern Rockies may see below normal highs especially on Sun. Farther eastward the gradual development of upper troughing will help to push an initial wavy front over the northern Plains southeastward with time. As a result the focus for areas of potentially heavy/strong convection will shift from the upper MS Valley/Great Lakes/north-central Plains toward/into the eastern-southern states. Some rainfall is also possible with another front that may drop into the northern states late in the period but less available moisture should lead to lower totals than with the activity earlier in the period. Ahead of these fronts early in the week, portions of the South and East may see scattered and generally light-moderate showers/thunderstorms. For temperatures east of the Rockies expect areas from the Great Lakes into the Northeast to see the warmest readings versus normal (especially for morning lows) in the Sun-Tue time frame. The expected pattern evolution should yield near to slightly below normal highs over a majority of the East by next Wed-Thu. 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