Megafires in a Warming World: What Wildfire Risk Factors Led to California’s Largest Recorded Wildfire

Massive wildfires and extreme fire behavior are becoming more frequent across the westernUnited States, creating a need to better understand how megafire behavior will evolve in our warmingworld. Here, the fire spread model Prometheus is used to simulate the initial explosive growth ofthe 2020 August Complex, which occurred in northern California (CA) mixed conifer forests. Hightemperatures, low relative humidity, and daytime southerly winds were all highly correlated withextreme rates of modeled spread. Fine fuels reached very dry levels, which accelerated simulationgrowth and heightened fire heat release (HR). Model sensitivity tests indicate that fire growth andHR are most sensitive to aridity and fuel moisture content. Despite the impressive early observedgrowth of the fire, shifting the simulation ignition to a very dry September 2020 heatwave predicted a>50% increase in growth and HR, as well as increased nighttime fire activity. Detailed model analysesof how extreme fire behavior develops can help fire personnel prepare for problematic ignitions.