(Bloomberg) -- Hurricane Helene began causing havoc across parts of the Southeast US more than a day before it made landfall on Florida's west coast on Thursday night. That includes historic rains that spread as far north as the Carolinas, priming the area for catastrophic flooding as the storm moves inland Friday and over the weekend.
Helene is currently a tropical depression churning across Kentucky, and the AP reports at least 21 are dead. The winds have dipped from a peak of 140 mph (225 kph) to roughly 35 mph. But flooding is still a major threat, in part because Helene’s rains got a head start.
The storm was steered north across the Gulf of Mexico by a dip in the jet stream known as an upper-level trough. That same atmospheric feature channeled moisture northward well ahead of landfall, dumping up to a foot of rain over the Carolinas and southern Appalachian Mountains on Wednesday and triggering flash floods.
Asheville, North Carolina, saw record-breaking daily rainfall on Wednesday and again on Thursday, for a total of nearly 10 inches (25 centimeters) over two days. As Helene's remnants pass through the region, up to six more inches will fall on waterlogged soil Friday, with total rainfall reaching 29 inches in some areas.
“If you're washing dishes and you've got a really saturated sponge, at some point the sponge can't take any more water,” said Peter Mullinax, a meteorologist at the US Weather Prediction Center in College Park, Maryland. “The ground is similar. As Helene moves inland, that's why there's such a significant threat for power outages and downed trees, which are loose in the ground. There's a higher risk of damage.”
It's not uncommon for a hurricane to generate rainfall over land while it's still churning across the open ocean, said Justin Lane, a meteorologist for the National Weather Service’s Greenville-Spartanburg office in North Carolina. But most storms don't unleash as much early rainfall as Helene did.
“We call these predecessor rainfall events,” Lane said. “Maybe you see two to four inches of rain. You usually don't see six to 12 inches, and that is really what made this event in particular so bad.”
The days-long heavy rains have pushed dams across the region to their limit. The NWS declared a flash flood emergency near the Lake Lure Dam in southern North Carolina Friday morning, warning residents below the dam to evacuate immediately as failure was "imminent.”
“I would be surprised if there are not multiple throughout this area,” said Brigadier General Daniel H. Hibner of the US Army Corps of Engineers. "It's not uncommon to see a dam failure during an event like this.”
Helene's rains have also caused local flooding and extensive power outages across Georgia, the Carolinas and the southern Appalachians. More than 3 million households were in the dark as of Friday afternoon, according to PowerOutage.us, in addition to nearly 900,000 without power in Florida, which took the brunt of Helene’s winds.
The French Broad River, which runs through Asheville and flows into Tennessee, is expected to crest at major flood stage Friday night, approaching a record set in 1916. At least 80 people died in those floods, which are still known locally as “the flood to end all floods.”
"We're not expecting much more rain today, but the larger rivers are still responding to the runoff," Lane said, adding that it would take days for the flood risk to pass.
A weakened Helene is expected to pivot west toward the Ohio Valley and parts of the lower Mississippi Valley later Friday. Large swaths of Illinois, Indiana, and Missouri are under a storm advisory. The NWS has also issued a flood warning for Nashville and much of middle Tennessee, with up to five inches of rain expected through Saturday. Extreme rains tied to hurricanes are a consequence of climate change, scientists say, as warmer air is capable of carrying more moisture that fuels more intense storms.
--With assistance from Brian K Sullivan and Ari Natter.
(Updates paragraph 2 with information about Helene’s current location and change from tropical storm to tropical depression.)
©2024 Bloomberg L.P.
