100-Year Rainstorms Defined

One of the more misleading phrases used in meteorology and hydrology is 100-year storm. The phrase implies that an intense rainstorm dubbed as an 100-year event, dropped rainfall totals heretofore unseen for 100 years, and not to be experienced again for another century. This is a logical, but incorrect conclusion to draw from the phrase. More precisely worded, a 100-year storm drops rainfall totals that had a one percent probability of occurring at that location that year. Encountering a 100-year storm on one day does nothing to change the probability of receiving the same amount of precipitation the very next day.

A better way to describe these unusual events is to refer to a one percent probability storm. However, the momentum created by repeated usage over time will assure that 100-year storm will remain in the public and scientific lexicon.

Intense rainfall events are often geographically isolated. Therefore, increased population density, improved precipitation monitoring networks, and radar-based precipitation estimation have increased the likelihood of capturing (measuring) heavy rain events. Also, improved communication allows for faster and more complete transfer of weather information. When the neighboring county is walloped by a 100-year storm, we hear about it quickly. Invariably we will vicariously "experience" the event and wonder why 100-year storms seem to be occurring every other week!

A 24-hour duration 100-year storm for most Minnesota communities is roughly six to seven inches. A comprehensive analysis of precipitation return period climatology is provided in NOAA Atlas 14, an on-line precipitation frequency information resource.


For more information contact: climate@umn.edu