100-Year Rainstorms Defined

A "100-year rainstorm" is the popularized name applied to a "1% probability storm,"and is defined as a storm that drops rainfall totals that have a one percent probability of occurring at that location in any year, and would, on average therefore, be expected to occur about once per century, assuming a stable climate.*

The concept of a 100-year storm may confuse some users because the phrase implies that an intense rainstorm dropped rainfall totals 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. Encountering a 100-year storm on one day at one location does nothing to change the probability of receiving the same amount of precipitation the very next day at the same location, or especially at a different one.

On one hand, increased population density, improved precipitation monitoring networks, and radar-based precipitation estimation, have all increased the likelihood of capturing (measuring) heavy rain events. We now hear about these events in near-real-time, and offices like this one (the DNR's State Climatology Office) often work quickly to verify these events in ways that simply were not possible in decades past. This certainly has added to a perception that 100-year storms have exploded in frequency.

On the other hand, rising global temperatures have increased evaporation rates off the world's oceans, putting more humidity in the air and thereby increasing the availability of "fuel" for precipitation. As a result, in Minnesota (and across much of the continent), extreme rainfall events are more common, more intense, and geographically larger on average than at any other time on record. We have evidence that "1% probability" rainstorms (or 100-year storms) may now have 1.33%, or even 2%, annual probabilities, meaning their approximate return periods would be down to 75 or even 50 years in some communities. These are still rare events that would only be expected once per lifetime on average, but even these small increases in frequency imply that a lot more intense, overwhelming rains are falling somewhere in Minnesota

According to NOAA Atlas 14, the leading precipitation frequency information resource, a 24-hour duration rainstorm with a 1% annual probability (a 100-year storm) for most Minnesota communities is roughly six to seven inches.

*In truth, the assumption of a stable climate has never been valid, because of normal climatic fluctuations not sampled by the initial baseline data. Now, in light of our changing climate and its increasing enhancement of precipitation processes, the assumption is widely understood by climatologists, hydrologists, engineers, and water resource managers to be badly flawed. The next generation of rainfall frequency/intensity estimates will be able to account for the changing (or "non-stationary") climate.     

Updated October 29, 2024