US floods: 40 trillion gallons of rain hit the South last week

That’s enough to fill the Dallas Cowboys’ stadium 51,000 times, or Lake Tahoe just once. If they focused only on the state of North Carolina, that amount of water would be 3.5 feet (more than 1 meter) deep. That’s enough to fill over 60 million Olympic-sized swimming pools.

“This is an astronomical amount of precipitation,” said Ed Clark, head of the National Oceanic and Atmospheric Administration’s National Water Center in Tuscaloosa, Alabama. “I haven’t seen anything in my 25 years of working with the weather service that is so geographically large and the sheer volume of water that fell from the sky.”

The damage from the floods is apocalyptic, forecasters said. More than 100 people died, according to officials.

Private meteorologist Ryan Maue, a former NOAA chief scientist, calculated the amount of rain using precipitation measurements made on 2.5-mile by 2.5-mile grids, measured by satellites and ground observations. He came up with 40 trillion gallons by Sunday for the eastern United States, with 20 trillion gallons of that hitting just Georgia, Tennessee, the Carolinas and Florida since Hurricane Helene.

Clark did the calculations independently and said the 40 trillion gallons (151 trillion liters) figure is accurate and, if anything, conservative. Maue said another 1 to 2 trillion gallons of rain had fallen, even though it was in Virginia, by his calculations.

Clark, who spends much of his work on water-depleting issues in the West, said to put the amount of rain in perspective, it’s more than twice the combined amount of water stored by two key Colorado River reservoirs: Lake Powell and Lake Mead.

Several forecasters said this is a combination of two, maybe three storm systems. Before hitting Helene, the rain had fallen heavily for days as a low-pressure system “turned off” the jet stream – which moves weather systems from west to east – and sat over the southeast. That carried a lot of warm water from the Gulf of Mexico. And a storm that fell just short of named status parked along North Carolina’s Atlantic coast, dumping up to 8 inches of rain, North Carolina State Climatologist Kathie Dello said.

Then add Helene, one of the biggest storms in two decades and one that rained a lot because it was young and moved quickly before hitting the Appalachians, said Kristen Corbosiero, a hurricane expert at the University at Albany.

“It wasn’t just a perfect storm, it was a combination of multiple storms that resulted in the enormous amount of rain,” Maue said. “This collected at high altitude, we’re talking 3,000 to 6,000 feet. And when you dump trillions of gallons on a mountain, it has to come down.”

The fact that these storms hit the mountains made it worse, and not just because of the runoff. The interaction between mountains and storm systems pulls more moisture out of the air, Clark, Maue and Corbosiero said.

North Carolina weather officials said their maximum was 31.33 inches in the small town of Busick. Mount Mitchell also received more than 2 feet of rain.

Before Hurricane Harvey in 2017, “I said to our colleagues, you know, I never thought in my career that we would be measuring rainfall in feet,” Clark said. “And after Harvey, Florence, the more isolated events in eastern Kentucky, parts of South Dakota. We witness events year after year where we measure rainfall in feet.”

Storms are getting wetter with climate change, Corbosiero and Dello said. A basic law of physics says that air holds nearly 4% more moisture for every degree Fahrenheit warmer (7% for every degree Celsius), and that the world has warmed by more than 2 degrees (1.2 degrees Celsius) since pre-industrial times.

Corbosiero said meteorologists are hotly debating how much of Helene is due to worsening climate change and how much is due to chance.

For Dello, the “imprints of climate change” were clear.

“We’ve seen the impact of tropical storms in western North Carolina. But these storms are wetter and these storms are warmer. And there would have been a time when a tropical storm would have headed into North Carolina and caused some rain and some damage, but not apocalyptic destruction. “