The National Hurricane Center predicted a chance of “numerous significant landslides” across southern Appalachia through Friday in the wake of Hurricane Helene. The Blue Ridge Mountains could see an additional 15 to 20 inches of rain as Helene approaches, with some areas seeing up to 30 inches or almost three feet of rain.
“Any time there’s heavy rain in a relatively short amount of time mudslides are a possibility,” said Phil Klotzbach, a hurricane researcher at Colorado State University.
After the storm makes landfall, Dr. Klotzbach said, it will move inland quickly, and because of its large size, the effects would be more far-reaching than that of a typical storm.
“The impacts are reaching pretty far inland,” he said, “which the Hurricane Center is emphasizing in its messaging.”
The areas at risk of extreme rainfall and flash flooding included parts of eastern Tennessee, northern Georgia, and where the western half of North Carolina meets South Carolina.
Asheville, N.C., was at the epicenter, said Carl Schreck, a climate scientist at North Carolina State University. But other cities like Greenville, Atlanta and Chattanooga, Tenn., were also at risk.
Between 3 p.m. on Wednesday and 7 a.m. on Thursday, Dr. Schreck had measured 6.5 inches of rain at his home in Asheville. That almost doubled his previous record of 3.5 inches over the past 12 years of measuring daily rainfall.
“In the past, when we’ve had major rainfall events there certainly have been landslides,” Dr. Schreck said.
The big plume of warm, moist air that produced so much rain is called a predecessor event, or a storm that can come before a hurricane still hundreds or thousands of miles away, according to Dr. Schreck. That storm soaked soils and could make landslides even more likely as Helene brings rain and hazardous gusts of winds on Thursday and Friday.
At a news conference on Thursday, Gov. Roy Cooper of North Carolina called Hurricane Helene an “unusually dangerous storm.”
He warned residents in the western part of the state that they could see more than a foot of rain, which was likely to “cause significant flash flooding, landslides, damaging debris flows, slope failures across steep terrain and river flooding.”
Over the last decade, flooding from extreme rainfall has been the deadliest cause of fatalities in the U.S. linked to tropical cyclones, according to the National Oceanic and Atmospheric Administration.
Intense rainfall is the most important factor for massive earth movements like mudslides and landslides, but topography matters, too. Soils slip more easily down steep hillsides, taking down rocks, trees and anything else in their path.
Plus, the large, fast-moving storm could bring extremely strong winds, which could uproot trees and further destabilize the landscape.
“Bringing hurricane force winds all the way up to southern Appalachia is a pretty unusual event,” said Daniel Brown, the branch chief for the hurricane specialist unit at the National Hurricane Center.
A new study from the U.S. Geological Survey recently showed that 44 percent of the United States could face the risk of a landslide.
Warning systems around the country for landslides are relatively rare. The few that exist are in areas like Southern California, which experiences landslides after wildfires, and southeast Alaska. After recent intense rains, deadly landslides hit a number of Alaskan towns.
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