The remnants of Hurricane Helene were not the only factor that contributed to the severity of the flooding that struck the mountain community of Asheville, North Carolina.
Several conditions in the region, including a precursor rain event and the topography of the land, gave rise to deadly flash flooding, experts told ABC News.
Hurricane Helene barreled into Florida’s Big Bend on Sept. 26 as a Category 4 hurricane, bringing 140 mph winds and a 15-foot storm surge to parts of the Gulf Coast. The hurricane exemplified the far-reaching impacts that a storm can have not only along the coast, but hundreds of miles inland. Following landfall, Helene tracked north, causing reported tornadoes in five states and dumping over 30 inches of rain in parts of North Carolina over the span of a few days.
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When large hurricanes form in the Gulf, they tend to cause a precursor rain event farther inland as the system’s large rain shield – defined by the National Weather Service as “a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye” – interacts with a cold front, Art DeGaetano, director of the NOAA Northeast Regional Climate Center at Cornell University, told ABC News.
The Asheville region had already received a deluge of rain in the days leading up to Helene, with more than a foot of rainfall in some spots, according to the North Carolina State Climate Office. As a result, the soil was saturated and rivers and streams filled to the brim by the time Helene, “the real whammy,” passed over the area, DeGaetano said. Once the French Broad River, which flows through Asheville, and its subsidiaries overflowed, the water rushed into nearby neighborhoods within minutes.
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While the phenomenon does not happen often, it has happened before in the region, DeGaetano said. A similar event occurred in Asheville in what is known as the “Great Flood of 1916,” in which 80 people died.
In addition, the sheer size of the storm system allowed areas to get hit with rain earlier and longer, Marshall Shepherd, director of the Atmospheric Sciences Program at the University of Georgia and former president of the American Meteorological Society, told ABC News.
The mountainous topography of the region also played a major role in the flooding, Yuh-Lang Lin, a professor of atmospheric science at North Carolina Agricultural and Technical State University, told ABC News.
Orographic lifting, a process that occurs when hills or mountains force air to rise and cool, caused water vapor to condense and additional precipitation to form as it combined with Helene’s tropical system, Lin said.
That heavy moisture was then “squeezed” out of the atmosphere and dumped over the Asheville region in a short amount of time, DeGaetano said. In hilly and mountainous terrains, rainfall typically is funneled into valleys, rivers and streams, DeGaetano said.
“That’s where we see this extreme flooding in those types of places,” Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center, told ABC News.
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Climate change also likely played a major role in the storm behavior displayed by Helene, DeGaetano said. The warm waters in the Gulf of Mexico helped the storm absorb even more moisture and intensify as it neared the coast, he said. The increased water vapor storage fueled the heavy precipitation experienced along Helene’s track.
Researchers are looking into whether an atmospheric river also contributed to the heavy precipitation, Shepherd said.
“You sort of had this multiple-whammy of the hurricane, that orographic lifting from the mountains and this atmospheric river,” he said. “Trillions of gallons of moisture coming in from the tropics.”
Human-amplified climate change is causing extreme rainfall events to become more frequent and more intense, according to the federal government’s Fifth National Climate Assessment, released in November, 2023.
Rising global temperatures are supercharging the water cycle and bringing heavier rainfall – and related flood risks – across the U.S. More intense extreme rainfall events increase the frequency and scale of flash flooding, in part because the influx of water is more than the current infrastructure was built to handle.
Most places on the East Coast have experienced an overall increase in rainfall due to climate change, DeGaetano said. In the Southeast, extreme precipitation events have increased by about 37% in recent decades, according to the National Climate Assessment.
