As hurricanes go, Sally was not especially powerful. Rated a Category 2 storm when it struck the Gulf Coast on Wednesday, it was soon downgraded. But climate change likely made it more dangerous by slowing it down and feeding it more moisture, setting it up to pummel the region with wind and catastrophic rainfall.
Sally was crawling at about 3 mph when its eye made landfall early Wednesday near Gulf Shores, Alabama, and was "inching its way inland” later in the day, the National Hurricane Center said. The slow movement, or stalling, of the storm led to staggering rain totals, with more than two feet in some areas by midmorning Wednesday and widespread flooding.
"When a storm moves slower, it lingers longer over the same location,” said Kimberly Wood, a geoscientist at Mississippi State University. "A rain rate of, say, an inch an hour — that’s not so bad if the rain only lasts 30 minutes. But if it lasts for half a day, that adds up quickly.”
Sally was not an isolated example of a stalling hurricane. "There is increasing evidence that storms are slowing down,” Wood said.
That evidence comes in part from a 2018 study that showed that hurricanes near the Gulf and Atlantic coasts were increasingly likely to stall. The study also found a clear signal of more local rainfall, said one of the authors, James P. Kossin, a researcher with the National Oceanic and Atmospheric Administration. "And it was associated with increasing frequency of stalled systems,” he said.
Climate change has also led to wetter storms, Wood said, because warmer air holds more moisture. Between the slowing speeds and increasing moisture, with storms like Sally "there’s a combination effect,” she said.
Researchers increasingly see a link between stalling of hurricanes and climate change. Rapid warming in the Arctic has reduced the difference in temperature between that region and the tropics, leading to a weakening and slowing of the jet stream and related winds that drive hurricanes’ forward movement.
Hurricanes also sometimes meander, Kossin said. Hurricane Harvey, which inundated Houston in 2017, moved back and forth over the area, increasing the deluge. Sally was heading due west, parallel to the coast, on Monday when it made a sudden right-angle turn to the north early Tuesday.
Such movements may also be linked to slowing atmospheric circulation, Kossin said. "You won’t really get meandering until you get a slow storm,” he said. "They don’t go zipping around like go-karts.”
While Sally’s winds were not as intense as the strongest hurricanes — maximum sustained speeds early Wednesday were about 105 mph, about 50 percent slower than a Category 5 storm — by lingering for longer, the storm may also have boosted storm surge, the wind-driven buildup of water that can quickly flood coastal areas, often with devastating results.
But storm surge can be influenced by many other factors, including the timing of tides and the shallowness of a bay or another body of water. In this case, Sally’s slow speed "contributed more to the extreme rainfall flooding than to the surge flooding,” said Rick Luettich, a professor at the University of North Carolina and a principal developer of the leading surge model used by forecasters.
Luettich said the storm’s surge was close to projections of about five feet. But another characteristic of some hurricanes that is linked to warmer oceans, the rapid strengthening of a storm before landfall, "gave the water a bigger push” than earlier forecasts called for, he said.
Hurricanes are not the only kind of storms affected by climate change, and not the only kind that can bring catastrophic flooding to the Gulf Coast or other regions. Record rain from a low-pressure system in August 2016, a large storm but one that did not rotate like a hurricane, led to floods in Baton Rouge. A gauge east of the city received 26.5 inches of rain in three days.
That storm prompted an attribution study, research that tries to determine the extent, if any, of climate change’s influence on an extreme weather event. It found that climate change had increased the likelihood of such a storm along the Gulf Coast in any given year by 40 percent since 1900. In the current climate, there is a 3 percent chance in any given year of a similar storm.
"The risk of extreme precipitation events in this region has gone up,” said Sarah Kapnick, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, who worked on the study.
"There’s a basic theoretical understanding underlying all of this,” Kapnick said. With warming "you get more water vapor in the sky.”
"So when you get these storms, be they hurricanes or summer storms, they have the potential to hold more water in them,” she added. "And that water has to go somewhere.”
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