Why Rare Storms Occur so Frequently and What to Do About It

Many people wonder why our cities have experienced so many X-year (fill in the blank for X) recurrence interval storms recently. The public has a hard time believing us when we report the third 10-year recurrence interval storm in seven years. We don't understand it ourselves. Do the statistics experts who develop precipitation frequency estimates really know what they are talking about? This paper identifies three reasons for the perceived higher than expected frequency of rare storms: 1) thinking regionally instead of considering just a single point, 2) rounding to the nearest X-year storm instead of using a threshold, and 3) ignoring storm duration. Numerical testing shows 71 to 700 percent increase in the perceived frequency of rare storms attributed to these reasons. Suggestions are offered for possible changes to rare storms classification to reduce public misperception.