Hurricane Helene, fueled by converging weather and climate ingredients, hammered the mountains of North Carolina in recent days with staggering amounts of rain and catastrophic flooding. In one of the worst natural disasters the region has ever seen, more than 20 to 30 inches of rain fell in less than three days, washing homes and roads away and virtually destroying entire towns while cutting large areas off from power and communications.

Flooding from tropical systems is common in mountainous regions, even those as far inland as western North Carolina. But in this case, the devastating drenching was exacerbated primarily by three factors: 1) heavy rain that fell even before Helene made landfall; 2) the region’s steep terrain, which amplified the flood risk; and 3) record-setting levels of atmospheric moisture boosted by human-caused climate change.

The result was a rainfall event that should happen only once every 200 to 1,000 years and what appears to be the most rain ever recorded in the Appalachians from a tropical system.

One-two punch of heavy rain

The historic deluge began in western North Carolina on Wednesday afternoon into evening, more than 30 hours before the core of Helene came through, because of what’s known as a “predecessor rain event.” That is when moisture streaming hundreds of miles out ahead of the storm interacts with a dip in the jet stream to generate an initial round of heavy rain.

Advertisement

The storm itself doesn’t always pass over the same area as the predecessor event, but when it does, it can be disastrous.

“The really bad scenario is where … the tropical cyclone passes over that same area a day or two later,” said Russ Schumacher, a professor of atmospheric science at Colorado State University. “The most impacts come from situations like this.”

After 8 to 12 inches of predecessor rainfall, the core of Helene plowed through the same region with a second round of torrential rain, on Friday, in areas where soils were already saturated and waterways swollen. That pushed rain totals as high as 20 to 30 inches — as much rain in less than three days as would typically fall in three to five Septembers.

Terrain intensified the rain and flooding

Some of the most devastating hurricane floods have occurred in the mountains, including flooding in Vermont from Hurricane Irene in 2011 and in Virginia from Hurricane Camille in 1969.

Advertisement

Western North Carolina, and specifically the Asheville area, is no stranger to tropical flooding, either. The remnants of two tropical systems combined for deadly flooding and mass destruction in 1916, while 2004 brought big floods from Hurricanes Frances and Ivan.

Mountains are particularly prone to severe flash flooding because of enhanced rainfall from upslope winds and water flowing down the steep terrain faster than on flatter land. As Helene approached from the south, the counterclockwise flow of wind around the storm’s low-pressure center resulted in persistent and increasingly strong winds from the southeast over western North Carolina.

“The southeast-facing slopes — those facing into the humid, low-level winds of Helene — forced that air to abruptly rise, cool to condensation, causing torrential rainout over the same locations,” said Jeff Halverson, a professor of meteorology at the University of Maryland at Baltimore County.

Advertisement

All of that water then concentrated into rapidly flowing, convergent streams, running off so fast that it could not be absorbed by soils. “What results is a powerfully erosive, overland flow of water, soil, rock, vegetation and man-made structures — the features of flash floods, mudflows and debris flows,” Halverson said.

Some areas witnessed violent landslides and floodwaters that rose many feet in an hour’s time. Vast volumes of water and debris ultimately landed in downstream rivers. Several — such as the French Broad River and Swannanoa River in Asheville — crested at their highest levels on record, surpassing marks that had stood for more than a century.

A record flow of water in the air

Rainfall associated with Helene amounted to 20 trillion gallons across North Carolina. The amount of moisture transported by Helene from the Gulf of Mexico into the region was more than 1.5 times the previous record and three times the amount of moisture moved by the most extreme atmospheric rivers — or jets of heavy precipitation — that frequently bombard the West Coast during winter.

Unusually warm ocean waters linked to human-caused climate change contributed to the extreme moisture levels. Tropical systems gain strength and capacity to transport moisture from warm upper ocean waters, and research has shown that storms occurring later in the season, like Helene, have a better chance of intensifying because, by then, warmer waters penetrate deeper into the ocean.

Advertisement

Helene developed and intensified over Gulf of Mexico waters experiencing a marine heat wave that warmed sea-surface temperatures as much as 3.5 to 4.5 degrees above average, while the upper layer of the ocean ranked as the second-warmest in more than 65 years of tracking.

One analysis published this week found human-caused climate change increased Helene’s rainfall by 50 percent; a second found the increase was around 20 percent.

Jason Samenow, Janice Kai Chen, Tim Meko and Laris Karklis contributed to this report.