Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1222 PM EDT Mon Aug 27 2018 Valid 12Z Thu Aug 30 2018 - 12Z Mon Sep 03 2018 ...Guidance and Uncertainty Assessment... Model guidance this morning shows reasonably good agreement on the overall synoptic pattern evolution across the U.S. through the medium range period. A high pressure ridge should continue to be more dominant from the central U.S. to the East Coast through the Labor Day weekend as a shortwave trough moves across the northern Plains. Meanwhile, a split flow is depicted for the western U.S. with the northern stream showing a tendency for amplification over the Pacific Northwest into the northern Rockies while a shortwave in the southern stream moves into California. Deterministic model solutions agree quite well through Day 5 Saturday and a compromise between the 06Z GFS and 00Z ECMWF yields good continuity with the previous WPC package. For Days 6 and 7, the deterministic ECMWF has been slowing down the eastward progression of the front/trough across the Northern Tier and appears to converge toward a solution for surface cyclogenesis over the northern High Plains into Alberta Canada by Labor Day. The GFS/GEFS has been a little more consistent on showing cyclogenesis in that vicinity. Therefore, more of the ensemble means from the 00Z EC and 06Z GEFS with a slight preference toward the 06Z GEFS were used to derive the Day 6 and 7 sea-level pressure field. ...Weather Pattern Highlights and Hazards... A continuing series of shortwave troughs will dig over the northeastern Pacific and split off the West Coast, but with more energetic flow and modest precipitation potential across the northwestern U.S., then progressing east-northeastward through southern Canada and the U.S. Northern Tier. Wavy frontal systems underneath will combine with deepened moisture and instability to fuel periods of heavy convection, focused from the eastern Plains/Mississippi Valley and Mid-South to the Great Lakes/Ohio Valley/Mid-Atlantic. Meanwhile, the potential for some Pacific energy to eventually work inland in separated southern stream flow by next weekend may renew monsoonal connections over the Southwest and South-central Great Basin/Rockies. Expect a threat for locally excessive, but less organized, thunderstorms/heavy downpours within the hot/humid sector in the southern/eastern U.S. which may align with meso-boundaries and local convergence effects. This potent late summer hot sector under ridging aloft will offer lingering widespread record high minimum temperatures into Thursday with high dew points into the upper 60s and low 70s. A moderating front will push through the Northeast and the mid-Atlantic thereafter, settling through the Carolinas. The hot sector over the U.S. southern tier will generally hold firm this week. Kong/Schichtel 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