Extended Forecast Discussion NWS Weather Prediction Center College Park MD 104 AM EDT Thu Aug 09 2018 Valid 12Z Sun Aug 12 2018 - 12Z Thu Aug 16 2018 ...Pattern Overview and Guidance/Predictability Assessment... Latest 12Z/18Z models and ensembles show very good clustering through the period for the CONUS, especially between the GFS/ECMWF. An upper low will temporarily meander near the Ohio Valley Sun-Tue before getting bumped northeastward by Canadian troughing. The 12Z UKMET/Canadian were just a bit displaced (quicker/slower, respectively) from the good GEFS/ECMWF ensemble clustering (with supporting deterministic runs) so opted to keep it simple and use the GFS/ECMWF and some of their ensemble means especially by next Wed/Thu as uncertainty increases. ...Weather Highlights and Hazards... Cooler temperatures will push through the Northwest and High Plains Sunday/Monday as the upper trough and surface cold front push through. A separated southwestward extension from the Ohio Valley upper low will help fuel organized rain/storms across Texas westward to the Four Corners region, especially NM/CO just to the east of the upper high in a region of favorable upper diffluence. Guidance continues to waver on placement but several inches of rain are likely near the Red River Valley (TX/OK) during the Sun-Thu time period. A wavy front will linger across the U.S. southern tier during the period which will help spark afternoon showers/storms and locally heavy rainfall in the warm/humid airmass. As the upper low slowly lifts toward New England, more widespread precipitation may be favored to its east side along the Appalachians to the I-95 corridor where the ground remains quite saturated in places (western Virginia through MD/PA into the Southern Tier of NY and Poconos in NJ. This could develop into a heavy rainfall threat considering the relatively slow system translation southerly fetch of ample moisture over several days. Closed lows in the summer can be underestimated in terms of precipitation intensity/amounts and longevity (see CSTAR research from UAlbany). Another frontal boundary will drop out of Canada toward Montana next Thursday or so with a return of 90+ degree temperatures across the High Plains. Fracasso 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