Hurricanes also sometimes meander, Dr. Kossin said. Hurricane Harvey 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-angled turn to the north early Tuesday.
Such movements may also be linked to slowing atmospheric circulation, Dr. 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 m.p.h., 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.
Dr. 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, La. 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.