St. Louis and Kentucky floods: How climate change intensified them

Biden issues disaster declaration as Kentucky flooding kills at least 16

Both flood disasters were triggered by 1-in-1,000-year rainfall events. Activated by the same atmospheric configuration, they exemplify the kind of dangerous weather that scientists believe will become more common as Earth warms.

Not all flash floods start the same way. Slow-moving tropical storms sometimes unleash days-long downpours over a large area, such as Hurricane Harvey in Texas in 2017 or hurricane florence in North Carolina next summer. In other cases, stagnant thunderstorms dump all their water in one unlucky spot.

This week’s flooding involved a parade of thunderstorms that passed over the same areas, like train cars along a track.

But all flash floods have one thing in common: so much rain falls that systems designed to safely divert the water are overwhelmed.

In the natural world, water is controlled by absorption into soils and discharge into streams and rivers. Man-made measures to regulate water include sewers and storm drains. But these systems have limits, depending on their design and location, and the intensity of the rainfall. Once these systems are overwhelmed, the water begins to drain in earnest.

Historic flooding in St. Louis kills at least 1, strands others

The longer it rains and the heavier the rain, the more likely flash floods will occur.

This week, the existing weather pattern over the Mississippi and Ohio valleys proved favorable for exceptionally heavy rain that exhibited unusual persistence.

It all started with a high pressure area over Bermuda and thunderstorms over the Gulf of Mexico. The storms injected water vapor from the warm gulf waters into the atmosphere, where it was blown north by winds blowing around that area of ​​high pressure. Every day storms were raging south of Louisiana, and a reliable flow pumped that tropical air inland.

However, the sweltering air’s journey hit a snag in a stationary weather front stretching from Kansas to Virginia, which overlaid a dome of excessively hot air stretching over the southern United States.

All atmospheric moisture began to accumulate near this stagnant limit, day after day. Eventually, the amount of moisture grew to near-record levels.

The waterlogged atmosphere, heated by the powerful late July sun, became loaded with storm fuel known as instability.

As the storms developed along the front, night after night, they drew energy from an atmosphere that was very unstable and very wet, and dumped rain with incredible ferocity. And because the high-altitude winds that dictate the movement of thunderstorms were blowing parallel to the front, the downpours moved over the same areas for hours, one after another.

Here’s how Hazard, Ky., received more than nine inches of rain in just 12 hours on Wednesday, and how more than 10 inches fell near St. Louis on Monday. That’s why flash flooding hit St. Louis again on Thursday.

In the valleys of eastern Kentucky, flooding was magnified by mountainous terrain, which funneled water to cities below, while sending river levels to record highs.

Understand 1000-year rainfall events and the role of climate change

This week’s weather pattern was so good at producing flash floods that the deluges in both St. Louis and many areas of eastern Kentucky qualified as 1,000-year rainfall events, a concept that can be difficult to grasp.

A thousand-year flood describes an amount of rain that has only a 0.1 percent chance of falling in any given year. Some places may see multiple 1,000-year events over 1,000 years; some might not see any.

Because the designation of a 1,000-year rainfall event is site-specific, the United States will often see many such events scattered throughout a given year.

But one limitation of the concept is that it assumes that the climate is stationary or unchanging. However, human-caused climate change is making such extreme, and statistically unlikely, precipitation events more common. A 1,000-year rain event probably doesn’t mean the same thing anymore as it did decades ago when the weather wasn’t as hot or humid.

According to the US government’s Fourth National Climate Assessment, the heaviest precipitation has intensified substantially across most of the country, including Kentucky and Missouri. This happens because a warmer atmosphere, capable of retaining more moisture, can produce more intense rains.

The assessment found that the amount of rain falling in the top 1 percent of events has increased by 27 percent in the Southeast and 42 percent in the Midwest over the past 60 years.

Both St. Louis and Hazard have seen increases in heavy rainfall in recent decades.

As temperatures continue to rise due to human-caused climate change, 1,000-year rain events and the tragedies they often leave behind are likely to become more common.

Jason Samenow contributed to this report.