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Record rain is hitting drought-stricken areas. That’s not good news.

A warmer climate is driving precipitation to higher extremes in both flooding and drought

A duck swims into White Rock Lake from a flooded sidewalk in Dallas on Aug. 22. (Emil Lippe for The Washington Post)

On Monday morning, the Dallas-Fort Worth area awoke to disaster. Rain was pouring down at the rate of 2 to 3 inches per hour. Highways became lethal lagoons, brooks became basins, and thousands of people scrambled to higher ground.

Just a day earlier, the city had been facing one of its worst droughts on record, with farmers forced to thin their herds as reservoirs rapidly shrank. Twenty-nine percent of the Lone Star State was encapsulated within a top-tier level 4 out of 4 “exceptional” drought. Very dry conditions took a heavy toll on crops and forced widespread water restrictions.

The extreme case of atmospheric caprice highlighted a growing issue plaguing communities across the United States and the world: weather whiplash.

This summer, several locations around the United States have experienced these wild, rapid swings from one weather extreme to another. About half of the country has undergone at least a moderate drought this summer. Parts of the West, the Midwest and Texas have experienced exceptional and historic drought conditions.

Then the storms came. On July 26 in St. Louis, a shocking 8.65 inches of rain fell to mark the city’s wettest day on record. The next day, in eastern Kentucky, rainfall rates topped 2 inches per hour and took the lives of 38 people. In August, eastern Illinois, Death Valley and Dallas also experienced significant or record-breaking rainfall. On Wednesday, flash flooding across central Mississippi swept away roads and prompted rescues.

Five 1,000-year rain events have struck the U.S. in five weeks. Why?

“It is unusual, especially on the extreme precipitation [and] flash flood side,” said Daniel Swain, a climate scientist at the University of California at Los Angeles. “They’re not just beating a historical record by a marginal amount, but just completely blowing right past it and then some.”

Yet he isn’t surprised: A warmer climate is driving precipitation to higher extremes in both flooding and drought.

“The increase in both extreme precipitation events and in these wild swings between extreme precipitation and extreme aridity — this is how most people and most ecosystems on Earth are experiencing climate change,” Swain said.

Two sides of the same coin

How can both drought and high-rain events result from climate change? Simple.

Warmer air can hold more water. In fact, for every degree Fahrenheit the atmosphere warms, the air can hold about 4 percent more water. Where there is moisture available, such as along the Gulf or East coasts, more moisture can be transported and dropped, leading to flooding and high-end precipitation totals.

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But where moisture is scarce, such as in the West, warmer air sucks humidity out of the ground. This desiccated landscape reinforces extreme heat, leading to drought and extreme wildfire behavior.

The U.N. Intergovernmental Panel on Climate Change highlighted this issue in a recent assessment report, writing that “aridification” and “extreme precipitation events that lead to severe flooding” are both byproducts of the warming climate.

“Whiplash events have always happened, but now we’re seeing the flips from one weather regime to another become more violent and disruptive...yet another clear signal that the climate crisis is with us now,” said Jennifer Francis, a senior scientist at Woodwell Climate Research Center, in an email.

There has also been a tendency for weather patterns to become “stuck,” stalling for longer. That may be why Dallas faced drought for months and was 11 inches behind for the year before this week’s flooding. Now, such precipitation extremes are more feast or famine.

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It may have to do with a wavier jet stream, which is believed to be shifting weather features west to east (in the Northern Hemisphere) more slowly. That allows for systems to stall. If a heat dome becomes entrenched in place, weeks of sunshine and dry weather can prevail. In all instances of thousand-year rain events in the West, Midwest and Texas this summer, a stalled frontal boundary was responsible for the deluges.

Make no mistake — when it comes to weather, getting a perfectly average day is atypical. Averages are just found by smoothing over the dips and crests in a random chaotic system. But when heat energy and entropy, or a bit of extra chaos, is added to that system, the dips and crests become much more extreme.

Although events have become more extreme, Francis said recent studies suggest that whiplash events haven’t become more common yet, but she added computer models “paint a clear picture of more frequent events if we continue to warm the globe by burning fossil fuels and destroying large tracts of forest.”

Flash flooding won’t cure a drought

Record flooding should fix a record drought, right? Not quite.

Water during a drought can help, but how fast and how much water falls matters.

During a drought, the ground dries and becomes less permeable. Top soils harden, which make it easier for water to run off. Drought also kills plants and leaves the ground bare, which further limits how much water the soil can absorb. When it rains, much of the water immediately runs off and doesn’t replenish the soils, aquifers or river flow beyond the initial burst.

“You get more instantaneous runoff, higher flashy flood flows on rivers and streams, but less of that water is soaking into the ground,” Swain said. “So you’re getting less soil moisture from the same amount of water.”

In fact, drought can actually lead to a greater risk of flooding. The dry ground hit with the rapid rainfall can promote runoff and trigger widespread flooding.

For instance, in Dallas, while the rainfall was desperately needed, most of what fell didn’t benefit the greater metro area. The event caused deadly flash flooding, but also almost all of the water that came down washed into a watershed that flows into Lake Livingston and eventually toward Houston. The National Weather Service in Fort Worth summed it up by writing “heavy rain, but wrong watershed.”

And last week, about 200 people were trapped for several hours in New Mexico’s Carlsbad Caverns National Park amid heavy flooding.

Swain said weather whiplash also means there are more dry days between the few rainy days — providing more opportunities for the water to evaporate back into the atmosphere. Even the water that does stick around may evaporate quickly, especially in a warming world.

“The soil moisture and the vegetation is still going to be responding in the long run as if there’s a severe drought, because in the long run, there still is,” Swain said.

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