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How Helene Wrought So Much Havoc, So Far From the Coasts
Hurricanes typically weaken over land. But if the ground is already wet from earlier rains, storms can receive an extra jolt that keeps them churning.
As rescue efforts continue across the Southeast after Hurricane Helene, scientists are starting to zero in on how the storm was able to cause devastation so far from the coasts, in places like western North Carolina that are less accustomed to hurricanes’ fury.
For scientists, Helene is 2024’s Katrina, a storm that will be studied for years to come, said Dev Niyogi, an earth and planetary sciences professor at the University of Texas at Austin. Hurricane Katrina ravaged New Orleans and the surrounding region in 2005, and it remains among the costliest and deadliest hurricanes in U.S. history.
Helene roared ashore in Florida last Thursday as a Category 4 hurricane then charged hundreds of miles inland, dumping trillions of gallons of rain along the way. The storm’s winds weakened over land, as hurricanes usually do because they no longer have access to the warm ocean waters that power them.
In Helene’s case, though, precursor rains had left the ground damp from Atlanta through the southern Appalachian region. This moisture could have given the storm an extra jolt of energy, particularly after a summer as hot as this year’s, Dr. Niyogi said.
“If you have wet and hot soil, then we are really priming the land” to juice up a storm, he said. Scientists call this phenomenon the brown-ocean effect, because it causes waterlogged soil to influence a storm in the same way the sea surface does.
Helene is the latest in a string of extreme rain and flood events that have inflicted misery across the globe this year.
World Weather Attribution, a scientific research collaboration, was preparing an analysis of Helene that would most likely be released next week, said one of the project’s founders, Friederike Otto, a climate scientist at Imperial College London.
The group planned to estimate how the storm’s rainfall, wind speed and other attributes might have been different in a world that humans hadn’t warmed by burning fossil fuels, Dr. Otto said. In recent weeks, World Weather Attribution has examined the influence of human-caused climate change on storms in Southeast Asia and Central and Eastern Europe.
Scientists say greenhouse warming will affect hurricanes and tropical storms in several ways as temperatures keep rising, according to the National Oceanic and Atmospheric Administration.
Storms are expected to cause more coastal flooding because of sea-level rise. They can deliver larger downpours because warmer air can carry more moisture. And they’re becoming more likely to intensify rapidly because they form over warmer seawater.
Helene strengthened to a Category 4 hurricane from a Category 1 storm in less than a day last week because of how hot the waters of the Gulf of Mexico have been. The number of hurricanes that ramped up so sharply before coming ashore in the United States has increased significantly in the past seven years.
Scientists do not expect the total number of hurricanes to increase globally as the planet heats up. But among the storms that do form, a larger share is expected to intensify to Category 4 or 5, according to NOAA.
Another factor that may have added to Helene’s destructiveness is the rain that fell in the Southeast last week, shortly before the storm swept through.
On Thursday, as several inches of rain were falling in Georgia, Marshall Shepherd, the director of the University of Georgia’s atmospheric sciences program, had a pressing concern: He was worried Georgians would think Helene had already hit.
In fact, the center of the storm was still over the Gulf of Mexico at that point. Dr. Shepherd feared people would let their guard down before the real wallop arrived.
He also realized that the predecessor rains might worsen Helene’s devastation in two ways. First, they would probably saturate the soil, making it more prone to flooding from the hurricane’s rains. And second, some of the moisture might evaporate during the storm, giving Helene additional energy to stay stable and spinning.
Dr. Shepherd and his colleagues are planning to study how much that second phenomenon, the brown-ocean effect, contributed to Helene’s catastrophic force.
Past research has found that evaporation over land helped keep Hurricane Ida in 2021, Tropical Storm Erin in 2007 and other tropical disturbances chugging along for longer. “Instead of just winding down, they just maintain their intensity and in some cases perhaps intensify,” Dr. Shepherd said.
A more detailed understanding of the brown-ocean effect could help forecasters better pinpoint the amount of rain that hurricanes will deliver so homeowners and infrastructure managers can prepare, Dr. Shepherd said. It should also help “shatter the notion” that hurricanes are hazards that only people living on coasts need to think about, he said.
“The fact that we’re just seeing these hurricanes penetrate farther inland as hurricanes, I think that is a harbinger of things perhaps to come in our state and even in the Appalachia region,” Dr. Shepherd said.
Raymond Zhong reports on climate and environmental issues for The Times. More about Raymond Zhong
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