One of the more confusing phrases used in meteorology and hydrology is "100-year storm". The phrase implies that an intense rainstorm dubbed as an "100-year" event brings 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. 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 your chances of seeing the same amount of precipitation the very next day.

Intense rainfalls are typically geographically isolated. Therefore, increased population density and improved precipitation monitoring networks have increased the likelihood of capturing (measuring) heavy rain events. Also, improved communication has allowed 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 "100-year storm" for most Minnesota communities is roughly six inches.

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URL: http://climate.umn.edu/doc/misc/100year.htm
Last modified: September 26, 2003