Characteristics of the landscape and atmosphere prior to two black summer Australian fires of 2019–20

• Wildfire potential changes with a range of environmental factors. • These factors define land surface, atmosphere and vegetation state. • We examine changes in such factors on a range of timescales. • Specifically, we compare variation in 2019–20 in Australia compared to prior years. • Multi-year drought, low winter rain and heatwaves combined in 2019–20 extremes. This work examines the interaction of atmospheric parameters with soil and fuel moisture content over scales of days, weeks to months, and years, across southern Australia. We demonstrate that consideration of different timeframes and regions is important for fire and land managers when making accurate assessments of fire risk. Our work extends research undertaken both prior and subsequent to the 2019–20 southern Australian fire season ( Black Summer ). The interaction between timescales is critical, as compounding of individual processes at the different scales occurs. This interaction was identified in this work in the cumulative effects of antecedent dry years, low winter rainfall and heatwaves during the 2019–20 fire season across southern Australia. The findings reinforce impressions of fire practitioners regarding the extremity of conditions antecedent to the 2019–20 fire season. The environmental parameters investigated show important differences between conditions in 2019–20 and those in immediately preceding warm seasons. Importantly, the datasets used in the project present a measurable and spatially coherent approach to estimating fire risk from observed and modelled soil and fuel moisture. Operational application of the datasets and approaches used here can assist in landscape and fire management, through better assessment of the vulnerability of regions to fire ignition and spread.