Hurricane Helene is now in the records of one of the most deadly storms to hit the United States in the last 50 years and the majority of the death toll so far comes from flooding and the resulting devastation caused by it in Western North Carolina.
The storm has claimed more than 230 lives, with about half reportedly coming from communities across Western North Carolina. And that number keeps rising as the search continues for those still missing.
Helene hit Western North Carolina as a tropical storm on Friday, Sept. 27, bringing inches of rain to an already saturated area causing rivers to burst from their banks, flooding communities and in some cases sweeping them downriver to become hazardous debris.
The question is, why was flooding so bad in this area?
There are several scientific theories on why this area saw double digits of rain in a four day stretch, one being the orographic effect.
What is the orographic effect?
The orographic effect occurs when air masses are forced to flow over high topography, aka the mountains, according to Penn State.
As these air masses rise over mountains, they cool and water vapor condenses. When this happens precipitation forms and is commonly concentrated on the windward side of mountains, this rainfall increases with elevation in the direction of storm tracks. When it continues to cool past the dewpoint, the amount of water vapor in the air can’t exceed the point of saturation, so it comes down as condensation and precipitation.
Helene was a fast-moving storm, but when it hit the mountains this orographic effect took place, according to University of South Carolina Geography Professor Cary Mock, who studies hurricane history as a geographer and climatologist.
“Normally, fast-moving storms are less of a rain hazard, but Helene was a big exception. In the southern Blue Ridge Mountains, Helene’s rain was enhanced by the terrain and what’s known as orographic uplift. When a storm is forced to rise up a mountainside, the air cools and condenses, dropping more precipitation,” Mock wrote in a recent piece for the school.
“In the mountains, that rainfall quickly funnels into streams and rivers. Asheville, North Carolina, a fast-growing city of about 95,000 residents, is located in a bowl in mountainous terrain. That left it and other nearby cities highly susceptible to high river runoff and extreme flooding. To make matters worse, the area was already saturated from a storm just ahead of Helene.”
Rainfall stalling over Western NC mountains ahead of Helene
Rainfall from Helene was already predicted to cause flooding in places no matter what, but in Western North Carolina a weather front stalled at the Appalachians ahead of Tropical Storm Helene which exacerbated the flooding, David Easterling, a rain expert with the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information in Asheville, said to USA TODAY.
How much rain fell in Western North Carolina due to Tropical Storm Helene?
While Helene dumped rain on the western portion of the Carolinas, the area had already been drenched the week before by storms. Here’s how many inches of rain communities saw between Tuesday, Sept. 24 and Saturday, Sept. 28, according to the National Weather Service.
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Busick in Yancey County had the most recorded rain in the period of time at 30.78 inches
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Spruce Pine: 24.12 inches
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Hendersonville: 21.96 inches
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Mountain Home: 17.09 inches
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Candler: 16.18 inches
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Tryon: 15.78 inches
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Grandfather Mountain: 15.42 inches
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Highlands: 14.86 inches
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Banner Elk: 14.85 inches
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Mills River: 13.26 inches
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Swannanoa: 13.21 inches
How high did rivers in Western NC get during Helene?
Rivers across the region still remain swollen a week after Helene first hit down on the region. On Sept. 27, rivers burst from their banks surpassing major flood stages by more than 10 feet in some areas. Rivers overtook communities across Western North Carolina, almost swallowing whole communities leaving nothing behind but mud and debris.
At its peak, the French Broad River in Fletcher crested at 30.31 feet on Sept. 27 as Helene moved through the area, according to the North Carolina State Climate Office out of North Carolina State University. In Asheville, the French Broad hit 24.67 feet, 1.5 feet above its previous highest crest. Further downstream at Blantyre, the river surpassed its 1916 crest of 27.1 feet to hit 27.38 feet before the gauge stopped reporting on Friday afternoon.
The Swannanoa River at Biltmore crested at 26.1 feet, more than five feet above what it did at its maximum in 1916 and slightly above the apparent 26-foot crest in April 1791, noted the climate office.
This article originally appeared on Asheville Citizen Times: Why did it flood so badly in North Carolina? Scientists point to this
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