Biden issues a disaster statement as Kentucky floods kill at least 16
Both flood disasters were spurred by 1 in 1,000 rainy events. Activated by the same atmospheric configuration, they exemplify the kind of dangerous weather conditions scientists think will become more common as the Earth warms.
Not all flash floods start the same. Sometimes, slow-moving tropical storms trigger downpours 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, single stalled thunderstorms dump all their water in an unfortunate place.
The floods this week resulted in a parade of thunderstorms that passed over the same areas, like railroad 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 in the soil and evacuation into streams and rivers. Man-made measures to regulate water include culverts and drains. But these systems have limitations, depending on their design and location, and the intensity of the rain. Once these systems are overwhelmed, the water begins to drain in earnest.
The historic flood in St. Louis kills at least 1, blocks others
The longer it rains and the heavier the rain, the more likely flash floods will occur.
This week, the weather pattern taking place over the Mississippi and Ohio valleys proved favorable to exceptionally heavy rains that have shown unusual persistence.
It all started with a high pressure zone over Bermuda and thunderstorms over the Gulf of Mexico. The storms injected water vapor from the warm waters of the gulf up into the atmosphere, where it was carried north by winds that blew around that high-pressure area. Every day, storms erupted south of Louisiana, and a reliable flow pumped that tropical air inland.
The soaking air journey hit a roadblock, however, in a stationary weather front that stretched from Kansas to Virginia, which covered a dome of excessively hot air swept over the southern United States.
All atmospheric moisture began to accumulate near this stalled boundary, day in and day out. Eventually, the amount of moisture rose to near record levels.
The water-soaked atmosphere, warmed by the mighty late-July sun, has become loaded with storm fuel known as instability.
As thunderstorms developed along the front, night after night, they drew energy from a very unstable and very humid atmosphere, and made the rain fall with incredible ferocity. And since the high-altitude winds that dictate the motion of the storms blew parallel to the front, the showers 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. This is why flash floods hit St. Louis again on Thursday.
In the valleys of eastern Kentucky, the flooding was amplified by the mountainous terrain, which channeled water into the cities below, as the level of the rivers reached all-time highs.
Understanding rain events of 1,000 years and the role of climate change
This week’s weather model was so good at producing flash floods that floods in both St. Louis and many areas of eastern Kentucky qualified as 1,000-year rain events, a concept that can be difficult to understand.
A millennial flood describes an amount of rain that is only 0.1% likely to fall in any given year. Some places may see more 1,000-year events in 1,000 years; some may not see any.
9 ”of rain in 12 hours in Hazard, KY is simply in its universe. To say that it is an event expected 1 in 1000 years, in a 20th century climate, is an understatement. But with climate change, what was then nearly impossible is now not only possible, it is probable. pic.twitter.com/yFV6PIZBIf
– Jeff Berardelli (@WeatherProf) July 28, 2022
Because the designation of a 1,000-year rain event is site-specific, the United States will often see many of these events spread across a given year.
But a limitation of the concept is that it assumes that the climate is stationary or immutable. Human-caused climate change, however, is making rainfall events so extreme and statistically unlikely more common. A 1,000-year rain event probably no longer means the same thing as decades ago, when the weather wasn’t as hot or humid.
According to the U.S. Government’s Fourth National Climate Assessment, heavier rainfall events have substantially intensified across much of the country, including Kentucky and Missouri. This is happening as a warmer atmosphere, capable of holding more moisture, can produce heavier rains.
The assessment found that the amount of rain falling in the top 1% of events has increased by 27% in the Southeast and 42% in the Midwest over the past 60 years.
Both St. Louis and Hazard have seen an increase in heavy rainfall in recent decades.
As temperatures continue to rise due to man-made climate change, rainy events of 1,000 years and the tragedies they often leave behind are likely to become more common.
Jason Samenow contributed to this report.