When Hurricane Florence struck the U.S. East Coast at 7:15AM ET this morning, it popped a squat near Wilmington, North Carolina. The hurricane — now downgraded to a tropical storm — is hovering over the area, unleashing torrential rainfall, and creeping towards the west at a glacial three miles per hour, according to the National Hurricane Center. That means the rain will continue to pound the same waterlogged ground — which, combined with storm surge, is leading to deadly flooding. So far, at least two people have died, according to Vox.
“Look at how slow this system is,” NHC director Ken Graham said in a Facebook Live briefing Friday morning. (When Graham talks about the storm’s speed, he means the pace at which it’s traveling, not the wind speed.) “Before that’s all said and done, it’s going to leave a wake of water, and river flooding, and a whole lot of impacts.”
We saw a similar leisurely pace with Hurricane Harvey, which dumped more than five feet of water on southeastern Texas, killing 68 people and causing so much damage that it was costlier than any hurricane except Katrina, the NHC says. So Florence’s slow speed means more danger for people in the storm’s path, as first reported by Kendra Pierre-Louis at The New York Times and Brian Resnick for Vox.
“Everything that comes with a hurricane sticks around longer,” says James Kossin, an atmospheric scientist at the National Oceanic and Atmospheric Administration. That means salt water flooding from storm surge, freshwater water flooding from rain, wind damage. “Right out of the gate, that’s not good.” Kossin’s research, published in June in the journal Nature, suggests that hurricanes have been slowing down over the past 70 years. Why? “That’s the million dollar question,” Kossin says.
In Florence’s case, there’s a high pressure system squatting to Florence’s north — which prevented Florence from moving up along the coast, and instead shunted it west onto land. That high pressure system is also exacerbating the storm’s stall. “Florence is caught between the winds wanting to blow it to the east, and that high pressure block preventing it from being blown in that direction,” says Charles Greene, a professor of Earth and Atmospheric Sciences at Cornell University. “It’s kind of stuck.”
The scientists The Verge spoke to were generally reluctant to connect the dots between Florence’s slow motion crawl through the Carolinas, and global warming. Rising global temperatures could be one explanation, but that particular link to climate change is more controversial than the links to other hurricane characteristics.
“When it comes to hurricanes and global warming, what’s a very robust finding is that they get more intense,” says Dim Coumou, who leads the Atmospheric Circulation & Extreme Weather research group at the Potsdam Institute for Climate Impact Research. Warmer air also can contain more water vapor, which can lead to more torrential rainfall, he says. But when it comes to a hurricane’s pace as it travels from place to place, Coumou says, “There is more uncertainty there.”
Broadly speaking, we do know that with global warming, temperatures at the poles are climbing more than at lower latitudes. And as the difference in temperatures between the Arctic and the equator diminishes, so could the pressure gradient that drives large scale air currents like the jet stream. That could mean slower movement of weather systems, including hurricanes like Florence, according to Kossin.
Other climate scientists are picking up similar patterns. Michael Mann, professor of atmospheric science at Penn State University, linked this amplified warming of the Arctic to jet stream behaviors “that are associated with the stalling of weather systems, including what we’ve seen with hurricanes over the past decade like Harvey, Irene, and now Florence,” Mann told The Verge in an email. The study, co-authored with Coumou, was published in the journal Scientific Reports in 2017.
Scientists are still figuring out the link — if there is a link — between hurricane travel time and climate change. On the one hand, Kossin says, it could be natural variability. On the other hand, he says, if there is “a human fingerprint on this linked to warming, then we would expect the slowing to continue — at least, that part of it caused by humans activity,” he says. But there’s certainly something going on. “It’s worrying that we see more stalling hurricanes and the slowdown of the circulation,” Coumou says. “That has to be understood.”