A 100-year flood or rainfall event is a phrase you've probably heard before, but it doesn't really mean what it sounds like, and that's important for staying safe when heavy rain threatens.

Here's the nerdy definition

The 100-year flood is a math concept. But we won't bury you in any advanced calculus, here.

All it means is a flood of a severity that has only a 1% chance of happening in any year at a given location, based on historical records.

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For example, if a 100-year flood at Anytown is 10 feet above flood stage, that means there's a 1% chance a flood at least that high will hit Anytown in any year.

According to the U.S. Geological Survey, this was developed in the 1960s as a metric to use for the National Flood Insurance Program.

What it doesn't mean

You probably picked up on how this term could be misleading.

Let's say your area just had a 100-year flood. So, you don't have to worry about one as bad or worse in your lifetime in that area, right?

Wrong.

Again, there's a 1% chance of it happening each year. As the graphic above shows, it's like you're rolling a 100-sided die each year. You may go many years without that unlucky roll. Or you could roll it two years in a row.

Such was the case recently along the Guadalupe River in central Texas.

Almost one year after the deadly July 4, 2025, flash flood, more deadly flash floods hit a stretch of the river in mid-July 2026.

In the town of Comfort, Texas, the July 2025 and 2026 flood crests were two of the five highest on record dating to 1869.

Other recent examples

There are many other examples of historic flood events occurring more frequently than what longer-term stats might calculate.

Hurricane Harvey's historic drenching in Houston and southeast Texas in August 2017 was their third 500-year flood event in three years, following the Memorial Day 2015 and Tax Day 2016 floods.

Ellicott City, Maryland, was hit by two devastating floods in 22 months, first on July 30, 2016, then again on May 27, 2018.

Climate change loading the dice

A warming climate may be contributing to heavier rainfall and higher-end flash flood events more often.

According to a Climate Central study, 90% of the U.S. cities they examined are seeing heavier rainfall rates now than 1970, a consequence of more moisture being able to exist in warmer air.

These heavier rain rates can lead to more frequent severe flash flood events of the type that statistics claim only have a 1% or less chance in any year, based on history.

So, the next time you hear the "100-year flood" phrase, remember that it's just a statistic.

The reality is that that type of event could happen again in your lifetime. And you may not have to wait that long to see it.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.​