How Helene became a ‘worst case scenario’
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These inland mountain communities are often safe from tropical storms. The cyclones that batter the U.S. southeastern coasts typically weaken as they come ashore. Many peter out before they reach a mountain town like Chimney Rock.
But this time, something different happened. Helene moved fast and carried its warm, moist air hundreds of miles inland into the Carolinas.
“It was a worst-case scenario for the type of tropical system that could deliver really extreme impacts that far inland,” said Gary Lackmann, professor of atmospheric sciences at N.C. State University.
Here’s what fueled Helene and caused so much devastation in the Appalachian Mountains.
The water in the air
Helene formed above unusually warm water in the Gulf of Mexico. As that water evaporated, it gave the storm the fuel it needed to rapidly intensify into a Category 4 hurricane and evolve into one of the widest cyclones to ever hit the United States.
“Water vapor is weather fuel, and it's controlled by the sea surface temperature,” Lackmann said. “So when you have record warm sea surface temperatures, you have record amounts of weather fuel.”
Climate change makes the freakishly hot conditions that fueled Helene’s growth more common.
“We won’t know the full estimated contributions from climate change until more thorough analysis is done. But this is a pattern we’re seeing around the world. Extreme rainfall events are becoming more common,” said Baker Perry, a professor of climatology at the University of Nevada at Reno, who previously taught at Appalachian State University.
The rain before the storm
Two days before Helene made landfall, record-setting rains were already starting to soak the Blue Ridge Mountains in Tennessee and North Carolina. A zone of low pressure along a front pulled water vapor up from the Gulf of Mexico into the mountains, where it quickly rose, condensed into storm clouds, and dumped heavy rain.
Those earlier rains “guaranteed that a lot of the water that came with Helene was not going to have anything to soak into, and so it was all going to be running down the surface to the nearest low-lying area and then just collect,” said Douglas Miller, a professor of atmospheric sciences at the University of North Carolina at Asheville.
They also created a wet, swampy environment that allowed Helene to cling to the last of its strength as it moved deep inland. Hurricanes weaken when they move over dry land — but the water vapor rising off the soaked ground in Helene’s path extended the storm’s life. “It’s slowing its demise,” Perry said. “It’s not strengthening, but it’s weakening slower than it otherwise would be.”
The water Helene brought
When Helene hit the mountains, it dumped all the remaining water it had collected from the Gulf of Mexico. Across the region, several months’ worth of rain fell between Sept. 25 and Sept. 27.
“This was a one-in-1,000 year event across a fairly large area in terms of the three-day rainfall totals,” Perry said.
The surge downhill
Unable to soak into the saturated soil, water flowed downhill. Along the way, it picked up dirt, boulders, trees and eventually cars, homes and slabs of asphalt. Solid ground became liquid and slid away.
“Eventually things break, and it just comes down in a muddy slurry of huge boulders, huge trees, and sometimes it can be moving 35 miles per hour down the slope of the mountain,” Miller said.
Rivers unleashed
The water flowing down the mountains funneled into the lowest ground it could find. Sometimes, it poured into streams and rivers. In other places, it flowed into valleys and hollers that were once bone dry.
“In the mountains, places flooded with torrents that don’t even have creeks. There was never even water there,” Perry said.
“When all this water is just coming down, what used to be a small creek, where you used to go find crawdads, now all of a sudden, it’s 10-15 feet deep,” Miller said. “And gravity and water combined together can really move quite a wall of force, very rapidly.”
Helene has become one of the deadliest hurricanes of the modern era. And now a vast operation is underway to provide relief in this difficult terrain.
About this story
Before and after images are from Nearmap, a location intelligence and aerial imagery provider. Mid-level water vapor satellite imagery is from GOES-16 Advanced Baseline Imager band 9 and processed with Google Earth Engine. Analysis of ERA5 Integrated Vapor Transport data by Ben Noll. Hourly precipitation data is from the NOAA Multiple-Radar Multiple-Sensor system and accessed via the Iowa Environmental Mesonet. National Water Center annual exceedance probability analysis is for the highest three-day rainfall from Sept. 23 to 28 using Stage IV data and NOAA Atlas 14. Terrain data is from USGS.
Editing by Monica Ulmanu.