How Hurricane Helene produced over 700 mm of rain in three days

Busick, NC reported 751.3 mm (29.58 inches) of rain between 8:00 a.m. September 25 and 10:00 a.m. September 27. At least 330 mm of rain fell in the college town of Boone, NC, home to Appalachian State University. Even the Atlanta metropolitan area received 300-350 mm of rain during the event.

Devastating floods

The end result was chaos. “All roads in western North Carolina should be considered closed,” the North Carolina Department of Transportation said during the storm Friday morning. Historic flooding plagued the region as massive runoff quickly overwhelmed streams, rivers and drainage systems.

The Swannanoa River in Biltmore, N.C., was one of numerous locations that experienced record-breaking river crests following the torrential rain. The river crested at 26.10 feet (7.96 m), exceeding flood stage by 16.10 feet (4.91 m) and breaking the previous record high of 20.70 feet (6.31 m) in July 1916.

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Mass rescues were required at Unicoi County Hospital in Erwin, Tennessee, after floodwaters completely inundated the building.

“As of 12:27 p.m., 54 people have been taken to the roof of the building and seven are still in lifeboats. The hospital was inundated by extremely dangerous and fast-moving water,” the hospital’s parent company reported on X on Friday.

Preceding rain events like Helene's are responsible for some of the worst flooding observed before and during landfall of a tropical system.