Exploring the "Surprise" 24 August 2016 Indiana–Ohio–Ontario Tornado Outbreak with Meso- and Storm-Scale Ensemble Simulations

On August 24th, 2016, at least 24 tornadoes developed over parts of Indiana, Ohio, and Ontario, shattering the daily record for the months of August and September in Indiana and Ohio (Guyer et al. 2016). However, the outbreak was notable and unusual not only for its large off-season tornado count, but also for being largely unanticipated in forecasts leading up to the event. Retrospective interrogation of previous- and same-day Convection-Allowing Model (CAM) guidance only weakly suggests the potential for substantial rotating storms over the region, and more generally that the greatest coverage of convection and severe potential would be centered much further west--over northern Missouri and northwest Illinois. Guyer et al. (2016) provided evidence that multiple subtle mesoscale factors, the most notable of which was the approach of a mesoscale convective vortex (MCV) from prior-day convection over Iowa and Illinois, combined to generate the outbreak. This suggests that a major reason for the “surprise” nature of the event may be that many of these factors were not present or well-resolved in the initial conditions of meso- and storm-scale numerical weather prediction (NWP) models leading up to the event.

To investigate the sensitivity of the 24 August 2016 tornado outbreak to initial condition uncertainty on the meso- and storm-scales, we perform a series of mesoscale ensemble analyses and forecasts of the event with the Weather Research and Forecasting (WRF) model coupled to the Data Assimilation Research Testbed (DART) Ensemble Kalman Filter (EnKF) system. We focus our analyses primarily on mesoscale factors, with careful attention paid to the influence of the aforementioned MCV. Additionally we will perform nested higher-resolution storm-resolving ensemble experiments. Contingent on the production of an accurate set of initial conditions on the mesoscale, this will allow us to get an idea of the inherent variability of the event on the storm scale. The overarching goal is to improve both our understanding and ability to forecast extreme mesoscale phenomena such as this remarkable tornado outbreak.