UC Merced

Short-duration precipitation extremes can threaten public safety and infrastructure by generating flash flooding and geophysical mass wasting events including mudslides and debris flows. Using two surface gauge-based precipitation datasets (1980-2024), we characterize the climatology of annual maximum subdaily precipitation and quantify trends across the western United States (WUS) – a topographically complex region with widely varying precipitation regimes prone to flash flooding. We find that 60.7% of WUS stations, including the vast majority of those located in the continental interior, typically experience 1-hr annual maximum precipitation (AMP) during summer and during the afternoon and evening hours (12:00-23:00 local time). Although most stations do not show statistically significant trends in 1-hr AMP intensity over the full period of record (1980-2024), a significant 10.3% domain-median increase in 1-hr AMP intensity was observed during 2000-2024. These changes largely result from seasonal maximum precipitation (SMP) increases during summer over the continental interior coinciding with a trend toward more favorable summer thermodynamic environments for short-duration precipitation extremes. We also report widespread though statistically insignificant increases in SMP intensity during winter and spring in California since 2000 coinciding with increased column water vapor. These results are suggestive of potential recent intensification of subdaily precipitation extremes in a warming climate in the WUS despite a backdrop of considerable internal variability.