The Meteorology of the Tathra Bushfire

Abstract

Abstract The meteorological conditions over the South Coast of New South Wales, Australia, are investigated on 18 March 2018, the day of the Tathra bushfire. We present an analysis of the event based on high-resolution (100- and 400-m grid-length) simulations with the Bureau of Meteorology’s ACCESS numerical weather prediction system and available observations. Through this analysis we find several mesoscale features that likely contributed to the extreme fire event. Key among these was the development of horizontal convective rolls, which emanated from inland and aided the fire’s spread toward Tathra. The rolls interacted with the terrain to produce complex regions of strongly ascending and descending air, likely accelerating the lofting of firebrands and potentially contributing to the significant lee-slope fire behavior observed. Mountain waves, specifically trapped lee waves, occurred on the day and are hypothesized to have contributed to the strong winds around the time the fire began. These waves may also have influenced conditions during the period of peak fire activity when the fire spotted across the Bega River and impacted Tathra. Finally, the passage of the cold front through the fireground was complex, with frontal regression observed at a nearby station and likely also through Tathra. We postulate that interactions between the strong prefrontal flow and the initially weak change resulted in highly variable and dangerous fire weather across the fireground for a significant period after the change initially occurred. Significance Statement The town of Tathra on the South Coast of New South Wales, Australia, was devastated on 18 March 2018, when a wildfire ignited in nearby bushland and quickly intensified to impact the town. Using high-resolution numerical weather simulations, we investigate the conditions that led to the extreme fire behavior. The simulations show that the fire ignited and intensified under highly variable conditions driven by complex interactions between the flow over nearby mountains and the passage of a strong cold front. This case study highlights the value of such models in understanding high-impact weather for the purpose of hazard preparedness and emergency response. Additionally, it contributes to a growing number of case studies that indicate the future direction of high-impact forecast services.