Why McCaysville saw major flooding again

A combination of tropical moisture, a slow-moving cold front, and a powerful flow of air just above the ground helped trigger another round of significant flooding in Fannin County, including hard-hit McCaysville.

It is the second time in roughly three months that the McCaysville area has faced major flooding.

Severe Weather Team 2 Meteorologist Brian Monahan said several ingredients came together to produce the extreme rainfall.

“A couple of things happening: a strong cold front, a slow-moving cold front, tropical air and moisture lifting very quickly just above the ground,” Monahan said.

He said a strong low- and mid-level jet stream was also moving across the region, with winds of 40 to 50 mph roughly 5,000 feet above the ground.

As that moisture-rich air pushed into the North Georgia mountains, the terrain helped force the air upward.

“It’s that air that was ascending the mountain ridges, and like a sponge, all of that moisture got wrung out over Fannin County and Gilmer County,” Monahan said.

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Beginning around 7 or 8 p.m., Monahan said the area experienced “wave after wave after wave of rain and storms” moving along the cold front.

The setup resulted in what meteorologists call training thunderstorms, when storms repeatedly move across the same location.

“The cold front, it acts like train tracks, and that’s why we call these training thunderstorms,” Monahan said. “They just keep moving over the same areas.”

Those storms remained focused along the front as they moved from south to north, dumping 6 to 10 inches of rain in about 12 hours in some areas.

Monahan said that amount of rainfall over such a short period qualifies as what is commonly called a 100-year rainfall event, meaning it has about a 1% chance of occurring in any given year at a particular location.

“That amount of rain in just the span of 12 hours” led to record crests on the Toccoa River and flooding in McCaysville, Monahan said.