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Wednesday, August 11, 2021

SPC Aug 11, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1127 AM CDT Wed Aug 11 2021 Valid 111630Z - 121200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO EARLY TONIGHT FROM CENTRAL/SOUTHEAST WI ACROSS NORTHERN IL TO LOWER MI AND NORTHERN IN/NORTHWESTERN OH... ...SUMMARY... Severe thunderstorms are likely across the Midwest and Great Lakes, especially across portions of Wisconsin, Illinois, and Michigan this afternoon and evening. A few severe storms capable of producing wind damage will also be possible from Virginia into western Pennsylvania and New York. ...WI area this afternoon/evening... A lead, convectively-enhanced midlevel trough will eject eastward over the lower Great Lakes today, in advance of a larger-scale trough that will progress across the upper Great Lakes. An associated surface cold front will move southeastward into WI/IA through this evening, and a very moist/unstable air mass in the low levels will spread northeastward into WI in advance of the front. Scattered supercell development is expected by early-mid afternoon across central WI as convective inhibition weakens in the warm sector, and storms will subsequently move east-southeastward and develop southwestward toward eastern IA. Extreme buoyancy is expected ahead of the front from IA into WI, where MLCAPE will likely reach 4000-5000 J/kg, where mid-upper 70s boundary-layer dewpoints underlie midlevel lapse rates near or greater than 8 C/km. The approach of the midlevel trough will sustain effective bulk shear of 40-50 kt, which will favor broken bands of supercells given substantial cross-frontal flow/shear vectors and storm motion off the front. These factors will favor an initial threat for very large hail and several tornadoes, given the moist boundary layer, effective SRH of 100-200 m2/s2, and a supercell storm mode. Swaths of damaging winds up to 75-80 mph will also be possible with any storm clustering or line segments that evolve from the initial supercells. There is some uncertainty in the severe threat this evening across southeast WI and northeast IL where the lingering influence of the morning convection should be most pronounced. ...Northern IL to the lower Great Lakes through tonight... An ongoing cluster of severe storms in northeast IL has produced a few severe gusts this morning, and there is potential for renewed development along the gust front as it encounters a destabilizing boundary layer across northern IN and southern Lower MI. Strong buoyancy and 30-35 kt westerly low-midlevel shear will likely be sufficient to maintain a damaging-wind threat with the expanding convection through the afternoon. Areas farther east into northwest and north central OH should have time to destabilize in the wake of morning convection, and there is some potential for the storms to reach western NY and northwestern PA by late evening/early tonight. ...VA/MD this afternoon/evening... A subtle midlevel perturbation with ongoing convection is moving slowly eastward over WV. A diffuse surface lee trough and differential heating to the east into VA should support scattered afternoon thunderstorm development as surface temperatures exceed 90 F and dewpoints are maintained near or above 70 F. Though vertical shear will remain weak, thermodynamic profiles will favor occasional damaging winds with precipitation-loaded downdrafts, giving MLCAPE of 2000-3000 J/kg and DCAPE near 1200 J/kg. ..Thompson/Bentley.. 08/11/2021 Read more LIVE: spc.noaa.gov
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