J Grant
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The water in the air
Helene formed above unusually warm water in the Gulf of Mexico. As that water evaporated, it gave the storm the fuel it needed to rapidly intensify into a Category 4 hurricane and evolve into one of the widest cyclones to ever hit the United States.“Water vapor is weather fuel, and it's controlled by the sea surface temperature,” Lackmann said. “So when you have record warm sea surface temperatures, you have record amounts of weather fuel.”
Sept. 26, 2 p.m.
Replay
A map key for the following map of satellite water vapor imagery
500 MILES
Climate change makes the freakishly hot conditions that fueled Helene’s growth more common.
“We won’t know the full estimated contributions from climate change until more thorough analysis is done. But this is a pattern we’re seeing around the world. Extreme rainfall events are becoming more common,” said Baker Perry, a professor of climatology at the University of Nevada at Reno, who previously taught at Appalachian State University.
The rain before the storm
Two days before Helene made landfall, record-setting rains were already starting to soak the Blue Ridge Mountains in Tennessee and North Carolina. A zone of low pressure along a front pulled water vapor up from the Gulf of Mexico into the mountains, where it quickly rose, condensed into storm clouds, and dumped heavy rain.Those earlier rains “guaranteed that a lot of the water that came with Helene was not going to have anything to soak into, and so it was all going to be running down the surface to the nearest low-lying area and then just collect,” said Douglas Miller, a professor of atmospheric sciences at the University of North Carolina at Asheville.
They also created a wet, swampy environment that allowed Helene to cling to the last of its strength as it moved deep inland. Hurricanes weaken when they move over dry land — but the water vapor rising off the soaked ground in Helene’s path extended the storm’s life. “It’s slowing its demise,” Perry said. “It’s not strengthening, but it’s weakening slower than it otherwise would be.”