Summary of 20240926-20240927 Flash Flood Event

Tropical Cyclone Helene began organizing over the western Caribbean on the 23rd and 24th of September before rapidly intensifying as it moved north through the eastern Gulf of Mexico on the 25th and 26th. A plume of moisture extending from the intensifying storm interacted with a slow-moving cold front to produce a band of widespread heavy rain showers and embedded scattered thunderstorms over the southern Appalachians and vicinity on the 25th and 26th, resulting in a "predecessor" heavy rainfall and flash flooding event over these areas. Helene made landfall in the Florida Big Bend and moved quickly N/NE through Georgia before turning toward the N/NW once the remnant center reached northeast Georgia. Tropical rainbands around the center of Helene swept over areas with already-saturated soils over the mountains and foothills during the early morning of the 27th, resulting in extreme rainfall and record, catastrophic flash flooding across multiple basins with headwaters near the Blue Ridge escarpment. Much of the area roughly west of U.S. Highway 321 received storm total rainfall of 7 to 13 inches. However, areas along and near the Blue Ridge escarpment saw widespread totals of 14 to 20 inches, with locally higher amounts of 2 feet or more. The community of Celo, located in the South Toe Valley below the eastern slopes of Mt. Mitchell in Yancy County reported a little over 30 inches. Dozens of fatalities occurred across western North Carolina due to flash flooding.

Streams with the most devastating flooding included the Broad River, where dozens of structures were swept away near the Henderson/Rutherford County line, the Swannanoa River, where dozens of people wee killed and hundreds of structures destroyed from Black Mountain downstream to Asheville, and the Toe River system in Yancey and Mitchell Counties, where multiple fatalities and widespread structural damage occurred. Flooding along the French Broad River in Henderson, Buncombe, and Madison Counties exceeded records that were set during the Great Flood of 1916, resulting in destruction of a portion of the River Arts District in Asheville and much of downtown Marshall. Flooding of the Pigeon River in Haywood County exceeded even the catastrophe of Tropical Storm Fred only three years earlier (August 2021), with much of the downtown area of Canton devastated, and sections of I-40 washed out downstream in the Pigeon River gorge. (The table below reflects record and near-record crests that occurred at major stream gauges during Helene.) Hundreds of debris flows occurred across the North Carolina mountains, with several large events that further contributed to destruction and fatalities. This included a single event near Fairview in Buncombe County that killed 11 people.

Due to its quick movement, Helene was still a strong tropical storm when the wind field reached the western Carolinas and northeast Georgia. Frequent wind gusts of 55 to 70 mph, with occasional gusts of 80 mph...and likely as high as 100 mph across the higher elevations combined with saturated soils to produce widespread damage to trees and electrical infrastructure. Across the mountains, some east and southeast-facing slopes above 3000 feet saw canopy-loss of 80% or more. Hundreds of trees fell on houses, vehicles, and power lines throughout the area, resulting in multiple fatalities. Almost all customers in several counties in upstate South Carolina and the North Carolina mountains lost power for at least a brief period. Most people were without power for at least a couple of days, while thousands were without power for a week or more.

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