Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sun Aug 12 2018 Valid 12Z Wed Aug 15 2018 - 12Z Sun Aug 19 2018 ...Pattern Overview and Guidance/Predictability Assessment... The main westerlies settle over Canada while a series of slow moving features are primed to drift eastward across the nation underneath 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 development from the mid-MS Valley to the Great Lakes and Northeast/Canadian maritimes Wed into next weekend. Renewed troughing aloft could emerge into next weekend over the central U.S along with surface wave formation/focus. Ensemble spaghetti plots show general west-northwesterly flow and embedded shortwaves. 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 centered over the southwestern U.S. while ripples in the flow push inland overtop into the Pacific northwest next weekend. The WPC medium range product suite was primarily derived from a composite blend of the 06 UTC GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean. Increasing guidance spread lends favor to an ensemble based forecast approach. That said, the 00 UTC ECMWF does offer a bit more amplitude than other models and clusters better with ensembles. This seems consistent with the overall flow/recent flow history and WPC continuity. ...Weather Highlights and Hazards... Threats of daily bouts of moderate to locally heavy rainfall should focus from the south-central Plains and mid-MS Valley/Mid-South through the the OH Valley to the Lower Great Lakes/Northeast as deepened moisture pools near wavy fronts. Surrounding locations can expect less focused diurnally forced convection, but with some locally heavy downpours. Meanwhile, across AZ and the Southern High Plains/Rockies in favorable upper diffluent flow, monsoonal flow will drive periods of daily convection and some local runoff issues. Schichtel 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