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How two 1-in-1,000-year rain events hit the U.S. in two days

The same dangerous atmospheric setup — intensified by climate change — spurred the back-to-back devastating deluges in St. Louis and eastern Kentucky

A home sits almost completely submerged in Jackson, Ky., on July 28. (Arden S. Barnes for The Washington Post)

First, a record-breaking deluge engulfed St. Louis on Tuesday, killing one person. Then, Wednesday night, eastern Kentucky bore the brunt of a second onslaught of high water that swamped entire communities. At least 25 people have died, and the toll is expected to rise.

The back-to-back deluges unloaded double-digit rainfall totals and sent stream levels to record heights.

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Both flood disasters were spurred by 1-in-1,000-year rain events. Triggered by the same atmospheric setup, they exemplify the type of dangerous weather scientists project will become more common as the Earth warms.

How the flood occurred

Not all flash floods begin the same way. Sometimes, slow-moving tropical storms unleash downpours for days over a large area, such as Hurricane Harvey in Texas in 2017 or Hurricane Florence in North Carolina the next summer. In other instances, single stalled thunderstorms unload all of their water on one unfortunate location.

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

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

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

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The longer it rains, and the heavier that rain is, the more likely flash flooding becomes.

This week, the atmospheric pattern in place over the Mississippi and Ohio valleys proved supportive of exceptionally heavy rain that displayed unusual persistence.

It all began with a zone of high pressure over Bermuda and thunderstorms over the Gulf of Mexico. The storms injected water vapor from the warm gulf waters high into the atmosphere, where it was blown to the north by winds racing around that high-pressure zone. Every day, storms erupted south of Louisiana, and reliable flow pumped that tropical air inland.

The journey of the sopping air hit a roadblock, however, in a stationary weather front stretched from Kansas to Virginia, which overlaid a dome of excessively hot air sprawled over the Southern United States.

All of the atmospheric moisture began to pool near this stalled boundary, 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 storms developed along the front, evening after evening, they drew energy from an atmosphere that was very unstable and very wet, and they dropped rain with incredible ferocity. And because the high-altitude winds that dictate the motion of thunderstorms were blowing parallel to the front, the downpours moved over the same areas for hours, one after the other.

This is how Hazard, Ky., received more than nine inches of rain in just 12 hours Wednesday, and how more than 10 inches fell near St. Louis on Monday. It is why flash flooding again struck St. Louis on Thursday.

In the valleys of eastern Kentucky, the flooding was magnified by the mountainous terrain, which funneled water into the towns below, while sending river levels to all-time highs.

Understanding 1,000-year rain events and the role of climate change

This week’s atmospheric pattern was so good at producing flash flooding 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 understand.

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

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

But a limitation of the concept is that it assumes that the climate is stationary or unchanging. Human-caused climate change, however, is making such extreme — and statistically unlikely — precipitation events more common. A 1,000-year rain event probably no longer means the same thing it did decades ago when the climate wasn’t as warm or humid.

Historic floods are occurring more frequently as climate change brings heavier rainfall across the United States. (Video: John Farrell/The Washington Post)

According to the U.S. government’s Fourth National Climate Assessment, the heaviest precipitation events have intensified substantially across most of the country, including in Kentucky and Missouri. This is happening as a warmer atmosphere, capable of holding more moisture, can produce heavier rain.

The assessment found that the amount of rain that falls 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 intense rainfalls in the past few decades.

As temperatures continue to rise because of human-caused climate change, 1,000-year rain events, and the tragedies they so often leave behind, will probably become more common.

Jason Samenow contributed to this report.