Cascading Wildfire–Atmospheric River Hazards: Postfire Flood Risk after the 2025 Eaton Fire

Abstract. Wildfires and extreme precipitation events increasingly interact under a warming climate, yet their combined hydrologic impacts remain poorly characterized. In fire-prone regions such as Southern California, atmospheric rivers (ARs) can both promote wildfire through antecedent vegetation growth and trigger severe flooding in postfire landscapes. This study evaluates the compound flood risk associated with a strong AR event occurring after the 2025 Eaton Fire by examining both its likelihood and hydrologic consequences. Historical AR records were analyzed to assess the probability of sequential-year strong AR events, and an event-based hydrologic modeling framework (AGWA/KINEROS2) was used to characterize watershed response under prefire and postfire conditions using precipitation analogous to the February 2024 AR. Results indicate that strong AR events (Scale ≥ 3) occur with a mean annual frequency of 0.51 and a 38 % probability of recurrence in consecutive years, supporting the plausibility of a postfire AR scenario. Simulations show that wildfire-induced landscape changes amplify downstream flood response, increasing runoff volume by approximately 6 % and sediment yield by about 50 % in channels immediately below the burned area. Sequential storm pulses further enhanced postfire impacts, with the second flood peak increasing by ~30 % relative to prefire conditions. These findings demonstrate that wildfire and AR events act as interconnected hazards whose cascading effects can intensify flood and erosion risks. By linking event probability with watershed-scale hydrologic response, this study provides a framework for assessing postfire flood hazards under compound climate extremes and underscores the need for integrated risk management in fire-affected regions.