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Tuesday, September 28, 2021

SPC Sep 28, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0259 PM CDT Tue Sep 28 2021 Valid 282000Z - 291200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE PORTIONS OF THE MID-ATLANTIC AND THE SOUTHERN PLAINS... ...SUMMARY... Isolated damaging wind gusts and hail are possible late this afternoon and early evening across parts of the Mid-Atlantic and southern Plains regions. With the shortwave trough already moving through the Mid-Atlantic this afternoon, the potential for scattered appears low late this afternoon and evening. The Slight risk has been removed based on these observational trends. Isolated strong/severe storms are still possible along the cold front and perhaps along the Blue Ridge. The Marginal risk will be maintained for this potential. ..Wendt.. 09/28/2021 .PREV DISCUSSION... /ISSUED 1129 AM CDT Tue Sep 28 2021/ ...Mid-Atlantic this afternoon/evening... An initial midlevel shortwave trough near the PA/MD border will move east-southeastward to the Mid-Atlantic coastline by mid afternoon, as part of larger-scale trough amplification over the Northeast (in response to upstream height rises over the northern Plains). The timing of the lead wave is ahead of the primary diurnal cycle, and additional storm development late this afternoon/evening will likely depend on lift along the cold front which will move southeastward from PA into the Mid-Atlantic by this evening. In the wake of morning convection with the lead shortwave trough, gradual moistening and surface heating in cloud breaks ahead of the surface front will contribute to MLCAPE of 1000-1500 J/kg by late afternoon from VA into the Delmarva/southern NJ. The moderate buoyancy and effective bulk shear of 25-35 kt will support a mix of multicell clusters and isolated supercells. The strongest storms will be capable of producing damaging gusts given some storm organization with DCAPE near 750 J/kg and 30-40 kt midlevel flow for downward momentum transfer. Midlevel lapse rates near 8 C/km and relatively cool midlevel temperatures will also support a threat for occasional large hail. ...Southern Plains this afternoon/evening... A dryline will establish this afternoon along the lee trough from near the western OK border to the Edwards Plateau, east of a weak midlevel trough over the southern Rockies. Boundary-layer dewpoints increasing into the 60s from south-to-north and strong surface heating/mixing will support thunderstorm development along the dryline this afternoon/evening from TX into southwest OK. MLCAPE of 1500-2000 J/kg and effective bulk shear up to 30 kt will favor a mix of multicell clusters and transient supercell structures, with an attendant threat for isolated large hail and outflow gusts of 50-60 mph given steep low-level lapse rates and DCAPE . Read more LIVE: spc.noaa.gov
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