Why Did It Flood So Bad?

Wondering why Asheville NC flooded? It's basically a bowl. The lowest point surrounding mountains. Rain floods it. But there is more to the story...

Why was there so much rainfall in Asheville, Western North Carolina?

Helene produced such a large amount of rainfall due in part to a predecessor event, small (relative to a hurricane) regions of heavy rainfall produced ahead of a hurricane. These rainfalls produce a band of "deep tropical moisture" which, when mixed with a hurricane, cause significant rainfall.

Those predecessor rainfalls hit Western North Carolina on Wednesday and Thursday. According to USA TODAY, 10 inches of rain fell on Asheville, 8 inches on Tryon and 6 inches in Bristol-Johnson, Tennessee before Helene even reached the area.

Did climate change play a part in North Carolina flooding?

Climate change is partially to blame as well. A recent study from Princeton University found more hurricanes are undergoing rapid intensification, where wind speeds increase at least 35 miles per hour within a day and produce "significantly higher rainfall hazard levels." In North America, this occurs when the hurricane soaks up moisture from the Gulf of Mexico.

Princeton University researchers predict RI (Rapid Intensification) events will increase 10-30% for each degree of global temperature increase. "The changing climate will likely lead to more rapidly intensified cyclones and greater flooding potential in the coming years," the study states.

Warmer oceans fuel storms

So why do hurricanes bring more rain in a warmer climate? Evaporation intensifies as temperatures rise, and so does the transfer of heat from the oceans to the air. As the storms travel across warm oceans, they pull in more water vapor and heat. That means stronger wind, heavier rainfall and more flooding when the storms hit land.

It is estimated that more than 60+ people have died from these floods, and the number of casualties is expected to increase as search and rescue teams can visit more of the impacted areas as water levels decrease.

The Setup for Relentless Rain

At 5 pm on Wednesday, September 25, Hurricane Helene was at Category-1 strength with its center just north of Cancun, Mexico, more than 500 miles and 30 hours away from its eventual landfall along the coast of Florida.

But it was already raining in Asheville by then, as a line of slow-moving showers along a stalled cold front – fed by tropical moisture from the fringes of Helene to the south – had set up from Atlanta through the southern Appalachians.

By midnight on Thursday, the Asheville Airport totaled 4.09 inches, and streamflows were already running at daily record high levels in upstream, upslope parts of the French Broad River basin.

The rain continued all day on Thursday as the frontal boundary had barely moved and Helene’s outer rain bands were closing in, adding even more moisture to the mix. More than nine inches fell that day across southern Yancey County.

Satellite imagery of Tropical Storm Helene over the Carolinas on Friday, September 27 at 8:00 am. (Image by NOAA/GOES East)

As mountain streams became overrun with moisture, that water rushed down the rivers and into towns such as Asheville, all while the heaviest rain from Helene was just beginning to fall.

The storm’s impacts were especially long-lasting because of its massive size. It developed in a high-humidity environment over the warm Gulf of Mexico, which let it grow and strengthen unimpeded.

That also created a broad southeasterly circulation – with tropical storm-force winds extending more than 300 miles from the center as it impacted Florida – that pushed even more moisture up the saturated mountain slopes.

From the start of the precursor frontal showers on Wednesday evening to the heart of Helene moving through on Friday morning, it was one of the most incredible and impactful weather events our state has ever seen.

The Whitson Avenue bridge that crosses the Swannanoa River was heavily damaged by flooding caused by Hurricane Helene in Swannanoa, N.C.

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