Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Thu Aug 09 2018 Valid 12Z Sun Aug 12 2018 - 12Z Thu Aug 16 2018 ...Pattern Overview and Guidance/Predictability Assessment... The medium range period features a well forecasted upper closed low temporarily meandering near the Ohio Valley Sun-Tue before getting kicked northeastward by Canadian troughing late in the week. A separated southwestward extension of the Ohio Valley low will also linger across the Southern Plains region leading to a heavy rain threat for Texas/Oklahoma this weekend and into early next week. Out West, strong ridging should hold across the Southwest, while possible weak shortwaves round the top of the ridge across the Pacific Northwest and northern Intermountain West. The latest models and ensembles show good clustering through day 5 for much of the CONUS. By days 6-7, normal run-to-run discontinuities and timing/intensity differences arise, particularly with the operational models. The one thing to note is the 06z GFS looked quite different from its previous runs late in the period, particularly out west, as it maintained strong riding well into the Intermountain West. Its previous runs along with the ECMWF, and to some extend the GEFS/EC Mean ensembles (albeit somewhat washed out) indicate possible weak shortwave riding overtop the ridge. For this reason, opted to use the 00z run of the GFS rather than the 06z run to maintain a solution closer to consensus, as well as continuity with showing some semblance of weakness in the upper ridge across the West. This cycle of the medr progs used a blend of the well clustered 00z ECMWF/GFS with their respective ensemble means. ...Weather Highlights and Hazards... A relief to the Western U.S. heat is finally in sight this weekend across the Northwest and High Plains Sunday/Monday as upper troughing and a surface cold front move through. The upper trough/low over the Southern Plains will help fuel organized rain and storms across Texas and westward into the Southwest/Four Corners region, particularly along the favored terrain. Guidance continues to waver on exact placement, but several inches of rain are likely near the Red River Valley (west Central TX into OK) during the Sunday to Thursday time frame. Rainfall lifts into the Mississippi Valley and the Ohio Valley late in the week ahead of upper troughing across the region later in the week. Monsoonal moisture and possible locally heavy rainfall will continue across the Southwest. 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 within the warm and humid airmass. As the Ohio Valley 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 some places. This could develop into a heavy rainfall threat considering the relatively slow system translation and southerly fetch of ample moisture over the course of several days. Summertime closed lows can be underestimated in terms of precipitation intensity/amounts and longevity. 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. Santorelli/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