Why Hurricane Sally Could Bring a Deluge

Climate change is making hurricanes wetter, because as the atmosphere warms it can hold more moisture. But Hurricane Sally is expected to dump as much as two and a half feet of rain on parts of the Gulf Coast over the next few days, and such enormous amounts cannot be chalked up to increased atmospheric moisture alone.

On Tuesday, the National Hurricane Center reported that Sally’s translation speed, the rate at which it moves forward, was about 2 miles an hour, and that the storm was not expected to accelerate much as it moved northward in the Gulf of Mexico toward an expected landfall Wednesday. It was stalling, in effect, as it approached the Mississippi coast.

Hurricane Paulette, by contrast, was zipping along with a translation speed of more than 25 m.p.h. in the Atlantic on Tuesday after passing Bermuda two days before.

Sally’s slow movement is leading forecasters to predict a deluge. A slow-moving storm drops more rain over a given area, leading to higher totals.

Other recent hurricanes have also stalled. A year ago, Dorian crawled over the Bahamas for a day and a half, causing widespread destruction from wind and storm surge. And Harvey, perhaps the best-known, and most costly, example of stalling, was no longer a hurricane by the time it stalled near Houston in August 2017. It had been downgraded to a tropical storm, but still it inundated the city and surrounding communities with four feet or more of rain over several days.

Hurricanes are carried along and steered by large-scale winds in the atmosphere, and research suggests that this atmospheric circulation is slowing down, at least at certain times of the year. Hurricanes could be affected.

A 2018 study found that globally since the middle of the 20th century, translation speeds of hurricanes and tropical storms had decreased by about 10 percent. Another study that year that focused on Atlantic hurricanes found that the average speed of storms near the North American coast had slowed by more than 15 percent.

This study also found a statistically significant trend in greater coastal rainfall and linked it to the increase in storms that stall. It said the results could be linked to natural climate variability and made no claims that human-caused climate change was at work.

But other recent research suggests that global warming — specifically in the Arctic, which is warming much more rapidly than other regions — is playing a role in weakening atmospheric circulation and thus potentially affecting hurricane speed.

Studies by Michael E. Mann, a climate scientist at Penn State, and others suggest that increased Arctic warmth reduces the temperature differential between that region and the tropics. This leads to a slowing of the jet stream, which affects other circulation patterns in the tropics but also in mid-latitude areas like North America.

“Our work indicates that climate change is favoring this phenomenon,” Dr. Mann wrote in an email message. “It likely plays a role in the decreased translation speed of landfalling hurricanes.”

Arctic warming is leading to other changes in the jet stream, Dr. Mann wrote, that are affecting weather to the south, including hurricanes. From spring through fall, he said, there is a greater tendency for large southward dips or other meanders in the jet stream to stay in place for days.

As they become stationary, these changes in the jet stream pattern lock zones of air in place, which can lead to prolonged heat waves or other extreme weather. Dr. Mann said that one such zone, of high-pressure air over the Central United States, favored the stalling of Harvey around Houston.

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