How Helene and other storms dumped 40 trillion gallons of rain across the South

Private meteorologist Ryan Maue, a former NOAA chief scientist, the amount of rain is calculatedusing rainfall measurements on 2.5 mile x 2.5 mile grids measured by satellite and ground observations. He expected 40 trillion gallons by Sunday for the eastern United States, with 20 trillion gallons of that hitting only Georgia, Tennessee, the Carolinas and Florida from Hurricane Helene.

Clark did the calculations independently and said the 40 trillion gallons (151 trillion liters) figure was about right and rather conservative. Maue said perhaps 1 to 2 trillion gallons more rain has fallen since his calculations, mostly in Virginia.

Clark, who spends much of his work dealing with the West’s shrinking water supply, said to put the amount of rain in perspective, it was more than twice as much water as two major Colorado River basin reservoirs combined: Lake Powell and Lake Mead.

Several meteorologists said it was a combination of two, perhaps three, storm systems. Before Helene struck, it had been raining heavily for days because a low pressure area had “cut off” the jet stream – which moves weather systems from west to east – and came to a standstill over the Southeast. This brought in plenty of warm water from the Gulf of Mexico. And a storm that narrowly missed the named status parked along North Carolina’s Atlantic coast, dropping up to 20 inches of rain, North Carolina state climatologist Kathie Dello said.

Add to that Helene, one of the biggest storms in recent decades and one that held a lot of rain because it was young and moving quickly before hitting the Appalachian Mountains, said Kristen Corbosiero, a hurricane expert at the University at Albany.

“It wasn’t just one perfect storm, but a combination of several storms that resulted in the enormous amounts of rain,” Maue said. “This accumulated at high altitude, we’re talking 3,000 to 6,000 feet. And if you drop trillions of gallons on a mountain, it’s bound to sink.”

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

In a short analysis, not peer-reviewed, but using a method published in a study of the rainfall from Hurricane Harvey, Three scientists at the Department of Energy’s Lawrence Berkeley National Lab found that climate change caused 50% more precipitation in some parts of Georgia and the Carolinas during Helene.

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

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

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