Preconditioning Drought and Fire Weather Jointly Elevate Megafire Risk in the Western United States

Antecedent drought and fire weather operate across different timescales and jointly shape conditions conducive to large-fire growth. Nevertheless, their joint association with megafire risk is understudied. Here we quantify joint rarity for 19,761 wildfires across the western United States (WUS) during 1984--2024. Fires exposed to jointly rare antecedent drought and fire-weather environments had a megafire rate of 7.58%, whereas 4.88% of all fires were megafires. Among fires exposed to these environments, 46.7% would not have been tagged as extremes-driven under a univariate extreme threshold perspective. Additionally, the annual share of fires exposed to jointly rare antecedent drought and fire-weather environments increased over time and was positively correlated with annual megafire rates, while exposure trends and the climate configurations associated with jointly rare exposure varied regionally across the WUS, with the strongest exposure trend observed in the Southwest.