SPC AC 181251 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0751 AM CDT Fri Sep 18 2020 Valid 181300Z - 191200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Organized severe thunderstorms are unlikely today over the contiguous U.S. ...Synopsis... In mid/upper levels, a longwave trough over eastern North America will continue to foster broadly cyclonic flow across the upper Mississippi Valley, Great Lakes, central/northern Appalachians, and Northeast. Upstream, ridging will progress eastward from the central/northern Rockies across adjoining High Plains, as a prominent synoptic cyclone moves inland over the Pacific Northwest. The center of circulation -- now evident in moisture-channel imagery about 75 nm west of Cape Blanco OR -- should become more diffuse as it approaches the northern OR coast today, and as the cyclone changes to a strong open-wave trough. The axis should lie close to the coast from WA to central CA around 00Z, reaching eastern WA/OR and western NV by 12Z tomorrow. Meanwhile, a broad area of weak heights and embedded vorticity lobes will extend from Tropical Depression 20 in the western Gulf northeastward across LA toward the northeastern CONUS longwave trough. At the surface, a weak low-pressure area -- representing the feeble remnants of what once was Hurricane Sally -- was drawn at 11Z from offshore of the Hampton Roads area of VA southwestward along a frontal zone to the ILM area. Associated substantial convection has moved offshore, as will the entire remnant cyclone over the next few hours. The front extended southwestward across the Fl Panhandle to the northwestern Gulf. ...Inland Pacific Northwest... Isolated to widely scattered thunderstorms are expected through this evening over western parts of WA/OR, with some activity potentially forming east of the Cascades this afternoon and moving northeastward across central OR. The strongest among these may produce hail and/or gusts near severe limits; however the potential appears too conditional and/or isolated for categorical-level probabilities at this time. Two main mesoscale areas of interest for strong convective potential are: 1. The northern Cascades of northwestern WA. Large-scale lift/ cooling will overspread the area beneath the left-exit region of the strongly cyclonically curved, mid/upper-level jet streak accompanying the progressive cyclone/trough. The related steepening of lapse rates aloft, over a relatively moist boundary layer, may yield areas of 500-800 J/kg MLCAPE, amidst 35-45-kt effective-shear magnitudes. 2. From the Willamette Valley northeastward to north-central OR, across intervening Cascades. Greater low-level moisture -- but also more cloud cover/precip-related low-level instability concerns -- will exist over the valley, where convective coverage may be greater. Deep shear also should weaken with time this afternoon closer to the trough, even as midlevels cool/destabilize. Meanwhile, stronger heating, steeper low-level lapse rates, and increasing deep shear in parameter spaces favorable for supercells will spread across areas east of the Cascades. Moisture and convective coverage will be limited, however. ..Edwards/Broyles.. 09/18/2020 NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z