The St. Louis and Kentucky Floods: How Climate Change Exacerbated Them

Suspension

First, a record flood swept through St. Louis on Tuesday, killing one person. Then, on Wednesday evening, Eastern Kentucky bore the brunt of a second onslaught of rising waters that inundated entire communities. At least 16 people were killed and the death toll is expected to rise.

Plunging successors dump double-digit precipitation totals and send flow levels to record heights.

Biden issued a disaster declaration as the Kentucky floods killed at least 16 people

The two flood disasters were caused by one-year rain events every 1,000 years. Powered by the same atmospheric composition, they exemplify the kind of dangerous weather that scientists believe will become more common as the Earth warms.

Not all flash floods start the same way. Sometimes slow-moving tropical storms release heavy rain for days over a large area, such as Hurricane Harvey in Texas in 2017 or Hurricane Florence in North Carolina next summer. In other cases, one stunted thunderstorm dumps all of its water into one unfortunate place.

This week’s floods included a display of thunderstorms that passed over the same areas, like train cars along a track.

But flash floods all have one thing in common – so much rain falls that it overwhelms systems designed to safely divert water.

In the natural world, water is controlled by soil absorption and emptying into streams and rivers. Man-made measures to regulate water include canals and rainwater drains. But these systems have limitations, depending on their design, location and rain intensity. Once these systems are flooded, the water begins to run off in earnest.

Historic flooding in St. Louis killed at least one person, and caused the death of many more

The longer it rains, and the more intense the rainfall, the higher the likelihood of flash floods.

This week, the prevailing atmospheric pattern over the Mississippi and Ohio valleys proved to be supportive of exceptionally heavy rain that showed 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 high into the atmosphere, where it was pushed north by winds racing around that high-pressure area. Every day, storms erupt south of Louisiana, and the reliable flow pumps that tropical air inland.

The fresh air ride hit a roadblock, however, in a steady weather front stretching from Kansas to Virginia, which shrouded a dome of excessively hot air scattered over the southern United States.

All the atmospheric moisture is beginning to gather near this discontinued boundary, day in and day out. Eventually, the amount of moisture grew to near-record levels.

The watery atmosphere, warmed by strong sunlight in late July, has become laden 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 fell rain with incredible ferocity. Because the high winds dictating the movement of thunderstorms were blowing parallel to the front, the torrential rains moved over the same areas for hours, one after the other.

This is how Hazard, Ky. , more than nine inches of rain in just 12 hours Wednesday, and how it fell more than 10 inches near St. Louis on Monday. Which is why flash floods occurred again in St. Louis on Thursday.

In the valleys of eastern Kentucky, floods were amplified by mountainous terrain, which carried water to the towns below, while sending river levels to all-time highs.

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

This week’s atmospheric pattern was so good at producing flash floods that floods in both St. Louis and many areas of eastern Kentucky are considered 1,000-year rain events, a concept that can be difficult to fathom.

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

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

But the limitation of the concept is that it assumes that climate is constant or unchanging. However, human-caused climate change is making extreme – and statistically unlikely – precipitation events more common. Perhaps a 1,000-year rain event no longer means the same thing it did decades ago when the climate wasn’t warm or humid.

Historic floods occur more frequently as climate change causes torrential rains across the United States. (Video: John Farrell/The Washington Post)

According to the US government’s Fourth National Climate Assessment, the heaviest rainfall events have intensified dramatically in most parts of the country, including Kentucky and Missouri. This happens because a warmer atmosphere, capable of holding more moisture, can produce heavier rain.

The assessment found that the amount of rain falling in the top 1 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 experienced increases in heavy rainfall in the past few decades.

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

Jason Samino contributed to this report.