SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0742 AM CDT Sun Oct 04 2020 Valid 041300Z - 051200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms are not expected across the U.S. through tonight. ...Synopsis... In mid/upper levels, a substantial mean trough over the eastern CONUS is forecast to deamplify through the period. This will occur as a series of nearly phased shortwaves -- now apparent in moisture-channel imagery from the upper Great Lakes across the middle Mississippi Valley to northeast TX -- eject eastward to the Atlantic Seaboard and southern Appalachians region. One of those shortwave perturbations -- now over IL -- will reach OH by 00Z, then reach the northern Mid-Atlantic overnight. At the surface, 11Z analysis showed a warm-frontal zone across he southern FL Peninsula near Lake Okeechobee, forecast to drift northward through the period. Isolated to scattered, episodic thunderstorms are expected again this period across the FL Peninsula and adjacent waters, as a rich moisture plume and weak but broad low-level lift occur within and south of the frontal zone. Further north, a low was drawn over IN between IND-HUF, with cold front southwestward over central AR, north-central TX, the TX Permian Basin, and southeastern NM. The low should remain weak and migrate eastward toward the northern Panhandle of WV around 00Z, with the front reaching eastern KY, middle TN, and southeast/south-central TX. ...Ohio Valley region... Isolated to widely scattered thunderstorms are possible from late morning to afternoon, shifting eastward to northeastward across the outlook area. Gusty winds are possible with the strongest activity, but instability and shear should be insufficient for an organized severe threat. A field of large-scale lift (warm advection and DCVA) related to the aforementioned IL shortwave trough is contributing to showers across parts of eastern IL and IN. From late morning into afternoon, as the leading edge of this ascent field combined with frontal lift and impinges on a modestly diurnally destabilizing warm sector, some of the convection may deepen enough for lightning production. Forecast soundings suggest 100-300 J/kg MLCAPE extending past the -20 deg C isotherm, with buoyancy extending (and supporting some thunder potential) behind the front. Though winds around 500 mb and higher levels will be strong, only the very upper reaches of the convective layer will extend into that regime, and only 20-30 kt effective- shear magnitudes are forecast. ..Edwards.. 10/04/2020
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