SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Wed Sep 22 2021 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF OHIO...WEST VIRGINIA AND WESTERN PENNSYLVANIA... ...SUMMARY... The greatest potential for damaging to severe thunderstorm winds and a few tornadoes today will be over parts of Ohio, West Virginia and western Pennsylvania. ...Synopsis... In mid/upper levels, a progressive northern-stream pattern is in place over most of the central/northern U.S. and nearby Canada. A well-defined shortwave trough -- apparent in moisture-channel imagery approaching the coastal Pacific Northwest -- will move inland by 18Z. As this perturbation pivots southeastward toward the northern Great Basin, general afternoon/early-evening thunderstorms (some with strong gusts) will be possible in the foregoing area of strengthening DCVA and marginal moisture, across parts of the northern Great basin. Downstream, an intense trough is evolving a closed cyclone aloft, over western/southern IL. The associated 500-mb low will deepen through the remainder of the period as the cyclone enlarges, with the center reaching the northwestern OH/southeastern Lower MI area by 12Z tomorrow. In response, the related surface low -- analyzed at 11Z near CMH -- will deepen and pivot mostly northward to east- central/northeastern OH by 00Z. At that time, the equatorward segment of the cold front should arch across southwestern PA, eastern WV, the western/central Carolinas, southeastern GA, the FL coastal bend, and the north-central/west-central Gulf. By 12Z, the surface low should be occluded and retrograded into a position close to the mid/upper cyclone center, with the cold front crossing central/eastern PA, eastern parts of VA/NC, and northern FL. ...Lake Erie to WV...southward to Carolinas... Scattered to locally numerous thunderstorms are expected to develop in a frontal band and move northward to northeastward across the area from late morning through afternoon. Isolated to widely scattered, relatively discrete cells also may form ahead of the front. Damaging gusts and a few tornadoes are possible. A pocket of relatively undisturbed boundary layer from WV across eastern OH and western PA will be subject to broken diurnal heating through cloud cover, with both the diabatic heating and low-level theta-e advection contributing to destabilization ahead of the front. Those factors should offset modest midlevel lapse rates enough, in an environment of already weak MLCINH, to help peak/ preconvective MLCAPE reach the 500-1200 J/kg across the categorical "slight" area. Meanwhile, mass response to the approaching deep-layer cyclogenesis and related height falls will strengthen mid/upper winds, deep shear and low-level hodographs across this region through the day. In a largely meridional-flow setup aloft (east and northeast of the deepening cyclone), backed surface winds with substantial easterly component are needed to optimize low-level shear, and are forecast, with effective SRH reaching the 150-250 J/kg range (locally higher), amidst effective-shear magnitudes of 40-45 kt. This will support a mixed-mode scenario with supercells, bows and LEWPs embedded in a near-frontal QLCS, and discrete supercell potential ahead of that band, until the QLCS enters a more-stable environment this evening over central PA/MD/northern VA where abundant antecedent clouds/ precip will have occurred for many hours -- and still may be ongoing upon frontal arrival. ...Central/southern Appalachians, Blue Ridge, Piedmont... A relative minimum in low-level instability is expected with southward extent from central WV across the remaining central/ southern Appalachians, while deep shear generally decreases eastward and southeastward. This should render the severe threat more isolated and conditional south of WV, mainly with the trailing part of the frontal line. The front will encounter a boundary layer over the western Carolinas characterized by strongest surface heating and richest low-level moisture in any of the outlook areas. Relatively weak deep-layer flow and related lack of shear will render the threat more poorly organized, localized and marginal in nature -- mainly in the form of damaging subsevere gusts (though an isolated 50+ kt gust cannot be ruled out). ..Edwards/Jewell.. 09/22/2021 Read more LIVE: spc.noaa.gov
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