Outside the city, severe flooding was widespread as Isaac sat defiantly on the coast. The National Hurricane Center expected the storm to drop up to 25 inches of rain in some areas. Officials said Wednesday night that they were working to evacuate up to 3,000 people from floodwaters in St. John the Baptist Parish, about 30 miles west of New Orleans. Tornado warnings were also in effect in several Mississippi counties.

Gov. Bobby Jindal of Louisiana said Wednesday that more than 4,000 people were in shelters across the state, and that 5,000 members of the National Guard had been deployed to help in response efforts. What is perhaps most remarkable about the storm is that there are still no reported fatalities in the United States, especially considering the degree to which it caught gulf residents by surprise.

“Initially, the storm only being a tropical storm instead of a hurricane, many people, especially the people who live down there, didn’t have a whole lot of concern,” said Deano Bonano, an aide to a parish councilman, referring to the town of Lafitte outside the levee. By Wednesday afternoon, the bayou that splits the town was rising so rapidly that scores, if not hundreds, of people were facing potentially days of being cut off from the world.

“I think everyone was surprised by this,” said Denny Mecham, the executive director of the new Ohr-O’Keefe Museum of Art in Biloxi, which was inches from taking in water. “They try to prepare you, but for people who are used to a Cat. 3 or Cat. 5, this doesn’t seem like much,” she said. “Everyone was saying, ‘We’ll be open by Thursday morning.’ Well, this is not how this one is turning out.”

The same calculus was relied upon in Plaquemines (pronounced PLAK-uh-men) Parish, whose residents are almost by definition hardy and self-reliant. Shrimpers, oystermen, ranchers and workers in the oil patch live together on this stretch of coastline divided by the Mississippi River nearly from head to foot, and they have been through it all: multiple hurricanes, the worst of the BP oil spill and a preference for occupations that are not generally associated with comfort and security. The parish was largely walled out of the federal levee system, much to the anger of the residents. They know what that means.