Why Extreme Rainfall Events Appear to Happen Much More Often Than Expected

The public often wonder why the cities they live in experience so many extreme rainfall events. Their occurrence is much more than would be expected based upon the recurrence interval estimated after the event by the engineers responsible for managing flood risk. Climate change is not the only answer. There are three additional factors contribute to the public perception that our frequency estimates are inaccurate. One factor is that recurrence intervals for rainfall are based upon point observations while the public thinks regionally, such as their metropolitan area. A second factor is rounding to the nearest recurrence interval instead of considering the published depth a floor value. A third factor is that a recurrence interval is tied to a storm duration while the public may ignore storm duration. This study quantifies the individual and joint decrease in extreme rainfall probability for a major metropolitan area caused by these three factors and discusses the implications to the public and the engineers who design and operate public infrastructure.