St. Louis and Kentucky Floods: How Climate Change Intensified Them

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First, a record deluge engulfed St. Louis on Tuesday, killing one person. Then, Wednesday night, eastern Kentucky felt the brunt of a second high water attack that submerged entire communities. At least 16 people have died and the toll is expected to rise.

Back-to-back downpours dumped double-digit rainfall totals and sent stream levels to record highs.

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Both floods were caused by 1 in 1,000 year rainfall events. Triggered by the same atmospheric pattern, they illustrate the kind of hazardous weather phenomena that scientists predict will become more common as the Earth warms.

Not all flash floods start the same. Sometimes slow-moving tropical storms unleash days-long downpours over a wide area, like Hurricane Harvey in Texas in 2017 or Hurricane Florence in North Carolina the following summer. In other cases, isolated thunderstorms stalled discharge all their water on an unfortunate place.

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

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

In the natural world, water is controlled by its absorption into soils and its discharge into streams and rivers. Man-made measures to regulate water include culverts and storm drains. But these systems have limitations, depending on their design and location, and the intensity of the rain. Once these systems are submerged, 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 become.

This week, the atmospheric model in place over the Mississippi and Ohio valleys was favorable for unusually heavy rains that showed unusual persistence.

It all started with an area of ​​high pressure 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 northward by winds that bypassed this high pressure zone. Every day, storms erupted south of Louisiana, and a reliable flow pumped that tropical air inland.

The sweltering air’s journey, however, hit a roadblock, in a stationary weather front stretching from Kansas to Virginia, which covered an excessively warm air dome extended over the southern United States.

All the atmospheric moisture began to accumulate near this blocked boundary, day after day. Eventually, the amount of humidity reached near-record levels.

The waterlogged atmosphere, warmed by the powerful late July sun, has charged with storm fuel known as instability.

As the thunderstorms developed across the front, night after night, they drew their energy from a very unstable and very humid atmosphere, and dropped the rain with incredible ferocity. And because the high-altitude winds that dictate the movement of thunderstorms were blowing parallel to the front, the showers moved over the same areas for hours, one after another.

That’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 a flash flood hit St. Louis again on Thursday.

In Kentucky’s eastern valleys, flooding was amplified by mountainous terrain, which funneled water into towns below, while raising river levels to all-time highs.

Understanding millennial rainfall events and the role of climate change

This week’s atmospheric pattern was so good at producing flash floods that the deluges in St. Louis and many parts of eastern Kentucky were called 1,000-year precipitation events, a concept that can be difficult to understand.

A millennial flood describes an amount of rain that has only a 0.1% chance of falling in any given year. Some locations may see multiple 1,000-year events in 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 throughout a given year.

But a limitation of the concept is that it assumes that the climate is stationary or unchanging. However, human-caused climate change is making these extreme – and statistically improbable – rainfall events more common. A 1,000-year rain event probably doesn’t mean the same as it did decades ago, when the climate wasn’t as hot or humid.

According to the U.S. government’s fourth National Climate Assessment, the heaviest rainfall has intensified significantly across most of the country, including Kentucky and Missouri. This happens because a warmer atmosphere, able to hold more moisture, can produce heavier rains.

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

Both St. Louis and Hazard have seen an increase in intense rainfall over the past few decades.

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

Jason Samenow contributed to this report.

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