A case study of South Australia’s severe thunderstorm and tornado outbreak 28 September 2016

An analysis of the South Australian severe thunderstorm and tornado outbreak of 28 September 2016, which produced at least seven tornadoes and contributed to a state-wide power outage, is presented here. Although challenging, prediction and understanding of tornadoes and other hazards associated with severe thunderstorms is very important to forecasters and to community and emergency services preplanning and preparedness. High-resolution deterministic and ensemble simulations of the event are conducted using the Australian Community Climate and Earth-System Simulator (ACCESS) model and the simulations are compared to radar and satellite observations. The deterministic simulation and two of the ensemble members show that the overall structure, orientation, intensity and timing of simulated thunderstorms is in good agreement with the observations. In the deterministic simulation, a hook-echo feature in the simulated reflectivity, indicating the presence of a mesocyclone, appeared at the time and location of one of the observed tornadoes. Two diagnostics were found to have good value for identifying tornado-formation risk. Updraft helicity successfully identified the potential for mesocyclone development, and the Okubo–Weiss parameter identified model-resolved mesocyclone rotation. The ensemble simulations show a wide range of outcomes for intensity, timing and structure of the event, as well as differences in potential for tornado formation. This emphasises the importance of ensemble simulations in forecasting severe weather and associated hazards, as ensembles identify a range of possible scenarios and the uncertainty, leading to improved guidance for forecasters and emergency services.