Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sun Aug 12 2018 Valid 12Z Wed Aug 15 2018 - 12Z Sun Aug 19 2018 ...Pattern Overview and Guidance/Predictability Assessment... With the primary corridor of the westerlies juxtaposed north of the international border with Canada, a series of slow moving features are primed to drift eastward across the nation next week. Pinpointing which shortwave will dominate is difficult to say given a lack of run to run model continuity in conjunction with the weak steering flows to contest with. While the initial impulse in the chain should be exiting off the mid-Atlantic coast on Wednesday, the focus shifts toward the next elongated trough pushing across the north-central U.S. The guidance continues to struggle with how amplified the flow will become as it moves downstream into the Ohio River Valley late in the work week. Ensemble spaghetti plots show a mixed bag of scenarios although a vast majority of these support some surface wave tracking toward the lower Great Lakes on Thursday night into Friday morning. Its slow forward motion should bring it across the vicinity of New England by Day 7/August 19. Difficult to say how deep its parent cyclone will be given the fluctuations in the guidance. Eventually renewed troughing could emerge next weekend across the central U.S. although this remains far from certain. Ensemble spaghetti plots depicts general west-northwesterly aloft with perhaps a shortwave embedded within it. Finally across western North America, multi-day ensemble comparisons have shown the ECMWF means to really back off on the poleward extent of the building ridge. The core of the upper ridge should stay confined to the southwestern U.S. while a ripple in the flow could push into the Pacific northwest next weekend. However, this remains the most volatile sector of the map so a lot can change in the upcoming days. The preferred solution utilized a multi-operational model blend through Day 4/August 16 led by the 18Z/12Z GFS and 12Z ECMWF with smaller contributions from the 12Z UKMET/CMC. Thereafter, building spread across western/central North America supported a more ensemble based forecast strategy. Generally compromised between the output from the 18Z GEFS/12Z NAEFS means with the 12Z ECMWF ensemble mean. Confidence remains near average although ensemble spaghetti plots show quite a bit of noise in the pattern next weekend which suggests slightly reduced certainty. ...Weather Highlights and Hazards... Given the persistence of shortwaves emerging out of the central U.S., daily rainfall threats should maintain slightly below average temperatures from the middle of the country eastward toward the Ohio/Tennessee River Valleys. Such departures should not be tremendous by any means with widespread numbers unlikely to exceed 5 degrees. Otherwise, the heat is expected to remain confined to areas of the western states. Above average heights will support toasty readings, generally around 5 to 10 degrees above climatology, particularly over the Pacific Northwest. Through Friday, this will also extend eastward into the Northern Rockies as well as the north-central states. Regarding precipitation threats, daily bouts of moderate to locally heavy rainfall are likely from Oklahoma/Kansas through Missouri and into the Ohio Valley/Lower Great Lakes region. Surrounding locations to this heavier swath can expect the usual diurnally forced convection although areal average amounts should be a bit lighter in nature. Across the Four Corners region, active monsoonal flow will drive periods of showers and thunderstorms which should subside late into the night. Rubin-Oster 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