TAMPA, Fla. (WFLA) — Earlier this week parts of Dallas, Texas, racked up 15 inches of rain in just 24 hours. Extreme rainfall is considered a once-in-1,000-year event. But what does that mean?

It means that if you were somehow able to live in Dallas for 1,000 years – based on a “normal” climate – you should only experience rain that intense once in your lifetime. But the weather is not normal anymore. We can now expect these epic floods to happen much more often because of a warmer climate.

Over the past month, the U.S. has had five distinct 1-in-1,000-year extreme rainfalls. Climate change does not cause these floods, but it does make them worse.


The reason is simple; warmer air holds more moisture and dumps more rain. The Earth has warmed 2 degrees Fahrenheit since 1900 and researchers say extreme rain events are now at least 15 percent wetter than they used to be.

An equation called the Clausius-Clapeyron equation says that for every 2 degrees F of warming, the atmosphere can hold 8% more moisture. But the rain produced is often more due to the fact that intense storms are very effective at assembling surrounding moisture and then efficiently raining it out. Some scientists describe it as Super Clausius-Clapeyron.

A 2017 paper found that climate change inflated hurricane Harvey’s biblical rain by 18 to 37 percent. And made the excessive rain totals at least 3.5 times (and up to 10 times) more likely.

Risser Et al. 2017/ Image:WFLA

A new paper released Thursday – the anniversary of Harvey’s landfall – puts hard numbers on the issue. The study found climate change increased the number of flooded homes in Houston by 50,000 – many in low-income neighborhoods.

Smiley Et al. 2022/ Image: WFLA

Along those same lines, a 2020 paper concluded that a large fraction of the $90 billion in damages caused by Harvey was due to climate change. The estimate is a lower bound of $30 billion and an upper bound of $67 billion. Essentially a warmer climate about doubled the economic toll.

Satellite view of Harvey’s Landall via CIMSS Satellite Blog

So what about here in the Tampa Bay Area? The map below shows how much rain would need to fall in 24 hours to cause a once-in-a-century rain event.

The answer is about 12 inches, but it varies from town to town. Along the Southeast FL coast, it is closer to 15″+.