The end-to-end convective hazard risk forecast process developed by the Australian Extreme Weather Desk for the South Australian 28 September 2016 tornado outbreak

During the afternoon and evening of 28 September 2016, one of the most significant severe thunderstorm outbreaks in recent decades impacted central and eastern parts of the Australian state of South Australia. Multiple supercell thunderstorms produced damaging to destructive wind gusts, including at least seven tornadoes, very large hailstones and locally intense rainfall. These supercell thunderstorms and tornadoes impacted the South Australian power network, contributing to a state-wide power outage leaving approximately 1.7 million people without power. The Australian Bureau of Meteorology National Operations Centre Extreme Weather Desk, established in May 2015 to provide “a national focus for extreme weather intelligence” and “enhanced severe weather capacity during periods of sustained demand”, developed a complete end-to-end forecast process for convection forecasting which included the development of guidance systems that inform national hazard risk forecasts of thunder, large hail, damaging wind gusts, heavy rainfall and tornado. The meteorology of this high-impact event is described on several scales, from the broad scale antecedent conditions to the synoptic and mesoscale development of the supercell thunderstorms and tornadoes. Very strong upper-level dynamics associated with cyclonic vorticity advection on the eastern side of a transient cyclonic PV-anomaly subsequently lead to explosive cyclogenesis of a surface low pressure system. Meanwhile, high-levels of moisture, exceptionally strong convergence along a cold front, and a highly unstable environment combined with a breaking Rossby wave and resulted in an atmosphere highly conducive to severe thunderstorms. United States National Weather Service Storm Prediction Centre normalised convective parameters highlighted environments conducive to tornadic supercell thunderstorms and large hail and demonstrated their application in the Southern Hemisphere. The Extreme Weather Desk's probabilistic hazard risk forecast products conveyed the risk of severe thunderstorms for this event. The products are the result of the amalgamation of best-practice guidance and continuous improvement through structured verification which provides an end-to-end forecast process.