Flooding rain events are becoming more common in many parts of the United States. But, the terms used to describe some of them, such as “100-year flood” or “500-year flood”, can be misleading.

While these phrases can make it sound like the flooding is cyclical, only happening once every 100 or 500 years, it is not. Storms are not on a schedule. Rather, these terms refer to statistical probabilities. For example, the “100-year flood” means there is a 1% chance of a flood of that magnitude happening in any given year in a given location. For the 500-year flood, the probability is 0.2%.

Given the right conditions, it is possible to see multiple 100-year or 500-year flood events in a relatively short period of time. Ellicott City, MD, for example, recently saw two 1000-year flood events only two years apart. For more information, watch the video below from the NWS.