A Mesoscale Analysis of the Development of Storms and Transition to Supercells during the Indiana and Ohio Tornado Outbreak of 24 August 2016

On 24 August 2016, an unexpected tornado outbreak struck portions of Indiana and Ohio, with 24 tornadoes occurring in a span of less than 7 hours in a region that had not been highlighted for tornadoes even hours before the first tornado occurred. Herein, a mesoscale analysis of the events leading up to and during this unexpected tornado outbreak is presented, revealing the presence of several mesoscale boundaries across the region. The first boundary was an outflow boundary that can be traced back to convection that developed the previous evening near Omaha, Nebraska. One round of elevated multicellular convection developed overnight ahead of this boundary owing to synoptic-scale ascent, while a second round formed along this boundary over Illinois on the morning of 24 August. The second boundary is a differential heating boundary that originated from anvil shading and latent cooling beneath the first round of elevated storms over eastern Illinois and Indiana. While the first round of elevated convection moved from Illinois into Indiana and dissipated, the second round transitioned from elevated multicellular convection into three discrete surface-based tornadic supercells, all of which produced EF-2 or stronger tornadoes. It is likely that reduced diabatic heating beneath the anvil resulted in less vertical mixing and preserved enhanced storm-relative helicity there into the afternoon hours. The development of supercells and production of significant tornadoes occurred from south to north, in tandem with the northward movement of this boundary during the afternoon. The third boundary was a westerly wind shift that triggered a second round of tornadic supercells across Indiana later that afternoon. An analysis of WSR-88D data is also presented, revealing the lack of a surging gust front with the early-day elevated convection, which likely also aided in the transition to a supercellular mode.