Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sat Aug 10 2019 Valid 12Z Tue Aug 13 2019 - 12Z Sat Aug 17 2019 ...Overview and Guidance/Predictability Assessment... The upper pattern at the beginning of the medium range period/Tue remains quasi-zonal, but with embedded shortwaves tracking through and some indication of troughing along the West Coast. As the week progresses, expect flow to transition to more amplified as a deeper trough digs into the West Coast and ridging strengthens in the Southern Plains Thu/Fri. This would likely lead to troughing downstream in the eastern U.S. By Sat it appears that energy from the western trough may split, with part moving eastward and part hanging back along the western U.S.--the 00Z and 06Z GFS and the 00Z ECMWF happen to be in good agreement with this, though the scenario has low predictability. But it does seem reasonable and similar to the pattern lately that energy will be more progressive when moving into the central U.S. from the West. A multi-model blend incorporating the 06Z GFS/GEFS mean and 00Z ECMWF/EC ensemble mean was able to be used, without leaning heavily on any one model run due to the continued smaller-scale uncertainties in the flow. Did lean away from the 00Z CMC after midweek for two reasons: first, the eastern trough is deeper than other guidance, and other guidance like the EC and EC mean has trended weaker with this troughing. Second, it is slower with the western trough by Thu, which means it does not move the energy eastward by Sat as expected. Overall this blend led to good continuity with the previous forecast issuance. ...Weather Highlights/Threats... The main rain maker in the medium range should be a frontal system moving southeastward through the eastern and central CONUS early in the period, stalling out over the Southeast late in the week. Widespread showers and thunderstorms are expected to move southward from the Northeast and Ohio Valley/Mid-Atlantic on Tuesday to focus in the Southeast/Gulf Coast from midweek onward as Gulf moisture streams into the vicinity of the front. Periods of rainfall and thunderstorms will also be possible ahead of shortwaves in the north-central U.S. Before the aforementioned front passes, warmer than average temperatures are forecast for the Southeast, Tennessee and Ohio Valleys, and Mid-Atlantic states. Additionally, the Southwest to Southern Plains should see hot temperatures through the period under the upper ridge. Thus, widespread record high minimum temperatures are expected in the southern tier of the CONUS and excessive heat is possible. On the other hand, with troughing making its way into the Northwest and shortwaves traversing through the Northern Plains, below average temperatures are forecast in those regions. Tate Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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