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Predicting where runoff‐generated debris flows might occur during rainfall on steep, recently burned terrain is challenging. Studies of mass‐movement processes in unburned areas indicate that event locations are well‐predicted by rainfall anomaly,R*, in which peak observed rainfall is normalized by local rainfall climatology. Here, we use remote and field methods to map debris flows triggered within the 2020 Dolan Fire burn area in coastal California, demonstrate that a short‐durationR*metric predicts debris‐flow occurrence more effectively than absolute peak intensity or longer‐duration rainfall metrics, and show that incorporating anR*criterion into an existing debris‐flow likelihood model can reduce false positive predictions and improve accuracy. We testR* at three other climatically distinct fires in California, demonstrating its utility for mapping likely debris‐flow locations in different climates. We also consider howR*can benefit postfire debris‐flow prediction given recent increases in climatological variability within individual burn perimeters. 

The size, frequency, and geographic scope of severe wildfires are expanding across the globe, including in the Western United States. Recently burned steeplands have an increased likelihood of debris flows, which pose hazards to downstream communities. The conditions for postfire debris‐flow initiation are commonly expressed as rainfall intensity‐duration thresholds, which can be estimated given sufficient observational history. However, the spread of wildfire across diverse climates poses a challenge for accurate threshold prediction in areas with limited observations. Studies of mass‐movement processes in unburned areas indicate that thresholds vary with local climate, such that higher rainfall rates are required for initiation in climates characterized by frequent intense rainfall. Here, we use three independent methods to test whether initiation of postfire runoff‐generated debris flows across the Western United States varies similarly with climate. Through the compilation of observed thresholds at various fires, analysis of the spatial density of observed debris flows, and quantification of feature importance at different spatial scales, we show that postfire debris‐flow initiation thresholds vary systematically with short‐duration rainfall‐intensity climatology. The predictive power of climatological data sets that are readily available before a fire occurs offers a much‐needed tool for hazard management in regions that are facing increased wildfire activity, have sparse observational history, and/or have limited resources for field‐based hazard assessment. Furthermore, if the observed variation in thresholds reflects long‐term adjustment of the landscape to local climate, rapid shifts in rainfall intensity related to climate change will likely induce spatially variable shifts in postfire debris‐flow likelihood.