As the storm rotated, dry air over Alabama and Georgia was swept into the system, mixing with the moist tropical air in the Gulf, he said. A pair of competing high-pressure areas to the east and to the north created a phenomenon called wind shear — winds at different levels of the atmosphere moving in very different directions — and that slowed the storm to a crawl.

“When you have dry air and shear, a lot of the rain impacts are displaced from the center” of the storm, Mr. Bannan said. And that makes them hard to predict, he said: “Some of the computer models really struggle with weaker systems and their outer bands.”

Even so, he said, Barry’s outer storm bands could become “reinvigorated and take off” on Sunday if the area warms up. Flash flooding was still a risk, and a strong storm line was also moving toward Baton Rouge from Lafayette. A tornado warning was issued for Denham Springs, on the eastern edge of Baton Rouge, at 8 a.m.

“Still not out of the woods yet,” Mr. Bannan said.

Transportation gets back to business almost as usual.

Transit systems and airports in southeastern Louisiana were largely returning to normal operations by midday Sunday, after suspensions or disruptions at the height of the storm.

Only one or two delays on arriving or departing flights were reported at Louis Armstrong International Airport in New Orleans or at Baton Rouge Metropolitan Airport.

“I can’t tell you how excited I am,” Kim Fisher, whose flight from Louis Armstrong had been canceled on Saturday, told the NBC affiliate in New Orleans. “I just can’t wait to get back to Michigan. Yes, I can’t wait, can’t wait to get home.”

The Coast Guard reopened the Port of New Orleans on Sunday, and the Regional Transportation Authority said that its buses and most of its ferries would be back to full operation by 1 p.m., though not its streetcars.