Hydroclimatic Rebound Drives Extreme Fire in California's Non-Forested Ecosystems.

The catastrophic Los Angeles Fires of January 2025 underscore the urgent need to understand the complex interplay between hydroclimatic variability and wildfire behavior. This study investigates how sequential wet and dry periods, hydroclimatic rebound events, create compounding environmental conditions that culminate in extreme fire events. Our results show that a cascade of moisture anomalies, from the atmosphere to vegetation health, precedes these fires by around 6-27 months. This is followed by a drying cascade 6 months before ignition that results in anomalously high and dry fuel loads conducive to fires. These patterns are confirmed when analyzing recent (2012-2025) extreme fire events in Mediterranean and Desert Californian biomes. We find hydroclimatic rebound as a key mechanism driving extreme wildfire risk, where moisture accumulation fuels vegetation growth that later dries into highly flammable fuel. In contrast, extreme fires in the fuel-rich Forested Mountain regions are less influenced by the moistening cascade and more impacted by prolonged drought conditions, which typically persist up to 11 months prior to fire occurrence. These insights improve fuel-informed operational fire forecasts for the January 2025 Los Angeles fires, particularly when year-specific fuel conditions are included. This underscores the value of incorporating long-memory variables to better anticipate extreme events in fuel-limited regions.