Extended Forecast Discussion NWS Weather Prediction Center College Park MD 119 AM EDT Wed Aug 15 2018 Valid 12Z Sat Aug 18 2018 - 12Z Wed Aug 22 2018 ...Pattern Overview and Preferences... Guidance continues to agree on an amplification of the large scale pattern toward a central/eastern North America trough and western US ridge. This evolution will be in response to a deep upper trough/low that develops to the south of the Alaska Peninsula and the downstream ridge which then builds over western Canada and vicinity. During Fri to Sat of the forecast models and ensembles are in good agreement on a wave of low pressure moving off the New England coast and trailing front moving south across the mid Atlantic. A shortwave aloft emerging into the High Plains this weekend spurs a defined surface wave to develop, with that wave merging with a wave causing a merger with low pressure moving northeast from the southern Plains to form a consolidated center of low pressure over the mid MS Valley by 12z Mon that moves northeast towards the Great Lakes late Mon and departing into Canada Tue. The 12z ECMWF was the strongest solution and results in heavier rain associated with the low pressure. The preference continues to be hedging with the 18Z GEFS/12Z ECMWF means for both the intensity of the system and resultant rainfall until more support for the stronger solutions develops. The ensemble means offer good agreement/continuity with low pressure reaching the Great Lakes by early day 5 Tue 21 Aug. The GFS/ECMWF/ensemble means center the high amplitude ridge over the southwest with above normal heights in the middle of next week across the northwest as the east Pacific ridge drifts inland. ...Weather Highlights and Hazards... New England low pressure provides widespread showers/storms Fri through Fri night, ending as the wave moves offshore Sat. Shortwave energy that likely reaches the Plains by Sun should provide focus for organized convection and locally heavy rain from the High Plains on Sat to across the central Plains to mid MS Valley on Sun. Low pressure moving northeast from the MS Valley towards the lakes drives showers/storms on Mon, and as the low pressure departs, showers/storms occur on the trailing cold front in the lower Lakes to the Appalachians and southeast Tue. Terrain/diurnal heating will promote showers/storms over parts of the Southwest with extreme southern areas likely to see the highest rainfall totals with greater available moisture. Florida and the Gulf Coast region should see diurnal showers/storms as well through the weekend, with more activity as the front approaches in the middle of next week. The large scale trough in the central US will bring an expanding area of below normal temperatures early next week. Currently expect the core of coolest high temperatures (10-15F below normal) to be centered over the north-central High Plains Sun-Mon. The cold front moves all the way to the Gulf coast so the southern Plains even gets a dose of cooler conditions in the middle of next week. Daytime highs over the Interior West will trend gradually warmer with time as ridging aloft rebuilds. Daytime highs are expected to be in the upper single digits to lower double digits above normal across interior WA/OR/ID. Petersen 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