Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sun Aug 04 2019 Valid 12Z Wed Aug 07 2019 - 12Z Sun Aug 11 2019 ...Overview and Guidance/Predictability Assessment... Guidance continues to show an eastward drift of the trough-ridge-trough pattern covering the eastern Pacific/North America domain. The slow progression of features will lead to a trough/upper low reaching the Northwest U.S. by next weekend, a Rockies/Four Corners ridge moving into the Plains, and the axis of the eastern trough eventually nearing the East Coast. The ejecting Pacific upper trough will bring a surface system into the West during the latter half of the period. Features within the eastern trough will reflect as a weakening cold front over the East late in the week followed by a better defined front that should progress east/south from the northern Plains Wed onward though stall over the south-central Plains. Most models and ensembles are fairly stable and similar with the mean pattern evolution. However some differences arise with specifics over the eastern Pacific and northwestern North America into central Canada by the late week/weekend time frame, with potential effects extending to the eastern mean trough toward the end of the period. Less confident aspects of individual solutions include the 00Z GFS becoming flatter than an overwhelming majority of ensemble guidance with the eastern trough aloft next weekend, the 00Z CMC becoming more amplified with western Canada troughing by late week and farther south with the upper low nearing the West Coast (as has been its habit the past couple days), and the 00Z ECMWF possibly being too quick to dislodge a 150-160W upper low over the weekend. Recent continuity/trends generally favor the middle to slower half of the envelope with the trough reaching the Northwest next weekend, adding some skepticism to the overall faster timing of the 00Z ECMWF over the eastern Pacific/Northwest U.S. toward the end of the period. In light of these considerations and typically declining confidence in specifics farther out in time, an operational model blend for day 3 Wed trended toward a model/mean blend featuring the 06Z GFS/00Z ECMWF and their ensemble means by the latter half of the period. The forecast held onto some 00Z ECMWF input through next Sun due to favorable comparison with consensus over the East, but with a low enough weight to emphasize the remaining solutions over the West. ...Weather Highlights/Threats... Guidance continues to show heavy rainfall potential over the central Plains/Mid-Mississippi Valley during the latter half of the week with focus provided by a couple decelerating/stalling fronts. The northern Plains and vicinity may see increased convection by next Sun depending on exact timing of the upper trough/surface system approaching from the Northwest states. This northwestern trough, and possibly some leading smaller-scale shortwaves, should produce one or more episodes of showers/storms over portions of the West. Expect monsoonal moisture to promote diurnal convection over the Four Corners states. Some activity may extend into the central High Plains, aided by a period of low level upslope flow. Aside from diurnal convection over the Southeast, showers/storms over the eastern U.S. should focus along two separate fronts crossing the region. Pockets of locally heavy rainfall will be possible over the East. Guidance hints a little more at parts of the Northeast and Southeast for such potential but the signal seems less clear than over some areas farther west. The approach/arrival of the eastern Pacific upper trough will bring a cooling trend to portions of the West with some areas likely seeing one or more days with highs 5-10F below normal. The fronts over the central U.S. will separate below normal readings over the northern and at times central Plains from hot weather over the southern Plains, with the heat also supported by the upper ridge settling over the region. On both sides of the spectrum anomalies should reach 5-10F from normal. Eastern U.S. temperatures should average close to normal for the five-day period with cold fronts providing some daily variability. Rausch 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