How did we get a dozen '1,000-year floods' in 3 days?

Both Texas and North Carolina experienced '1000-year floods' this week. How is that possible, and is the danger increasing?

After nearly 2 feet of rain fell in central Texas on July 4, causing flooding that killed more than 100 people, almost a foot of rain struck central North Carolina as Tropical Storm Chantal moved inland on July 7. Both are considered to be at least "1,000-year flood" events. But what does that mean, and how could we see so many events in such a short time period?

The terms researchers use to rank floods and heavy rainfall are complex, nuanced, and often confusing.

1,000-year floods, defined for insurance purposes

For home insurance purposes, a 100-year flood means that there is a 1 percent chance of that flood happening in any given year at your home's location. A 1,000-year flood would be a 0.1 percent chance, so in 1,000 years, you'd expect 1 such flood to occur. But statistics don't dictate when during that period the floods will occur, and the floods could happen in rapid succession without violating the rule.

These flood statistics are based on how often the nearest USGS river gauge (there are about 7,500 nationwide) would exceed a certain amount of water flowing past it, based on records at those stations that go back as far as 175 years.

1,000-year rainfall, for meteorologists

For meteorological purposes, when we say "1,000-year flood," we are really talking about the frequency of a certain amount of rainfall at one of tens of thousands of points that have weather station records going back to the 1800s. In meteorology, this is called the "return interval." Again, a 1,000-year rainfall return interval would be a 0.1 percent chance of rain that heavy, or something you could expect in 1,000 years. While that's not exactly the same as the flood definition, they are often used interchangeably.

There are tens of thousands of discrete points on land for which these rainfall amounts are determined, and each has its own 1,000-year amount. There's no reason that multiple locations in the United States couldn't have a 1,000-year rainfall at the same time. In fact, this week, half a dozen points in Texas and North Carolina both had 1,000-year rainfalls.

To complicate things further, each point has different time periods that define the floods, from minutes to hours to days. So, a location could set a 100-year rainfall for 24 hours while setting a 10-year rainfall for 1 hour, at the same time.

Those combinations produce millions of possible records to be broken, so it's not unusual to see multiple records broken every day.

How many 1,000-year rainfalls have there been this year?

Looking at 24-hour time periods of rainfall return intervals, Colorado State University says that 30 points in the United States have broken their 1,000-year rainfall amounts so far this year. There were 35 last year.

Between 2002 and 2024, there were hundreds of 1,000-year rainfall events for a 24-hour period. For 100-year rainfall, the threshold was exceeded thousands of times in every state monitored (Washington and Oregon are missing due to missing historical data there).

The most common months to break a 24-hour return interval are the summer months -- June, July and August -- because that's when the heaviest thunderstorms happen. September and October are also big months because of tropical storms and hurricanes.

Is climate change making flooding worse?

While Colorado State's database may not be long enough to make assumptions, ClimateCentral says that climate change is making flooding events worse than they would otherwise be. With increasing temperatures, the atmosphere can hold more moisture, which can make for heavier downpours. More rain falling in shorter periods makes flash flooding worse. In Austin, they say that rainfall intensity has increased by 19% between 1970 and 2024.

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