Tourists and residents alike appeared to have heeded that warning. Shortly after midnight in and around the French Quarter, streets normally packed with partiers were deserted, washed by sheets of rain and blown by winds that made hanging building signs swing wildly.
"Nobody is actually out here partying from what I've seen," said Jared Farrell, a parking valet for several hotels.
Tracy Smith, 26, a New Orleans resident who decided that she and her family would be safer at La Quinta hotel near the quarter than at home, ducked out shortly after midnight to gauge the storm's severity. Farrell yelled over to her to watch out for a restaurant sign that had become partially detached from a building and threatened to fly off.
Smith, a former deputy sheriff, was trapped for several days with about 100 inmates in a New Orleans jail during Hurricane Katrina, up to her waist in floodwaters. She is still haunted by the experience.
"That's why I was panicked for this storm," she said.
Tens of thousands of people were told to leave low-lying areas, including 700 patients of Louisiana nursing homes, but officials decided not to call for mass evacuations like those that preceded Katrina, which packed 135 mph winds in 2005.
Isaac also promised to test a New Orleans levee system bolstered after the catastrophic failures during Hurricane Katrina. But in a city that has already weathered Hurricane Gustav in 2008, calm prevailed.
"I feel safe," said Pamela Young, who settled in to her home in the Lower 9th Ward — a neighborhood devastated by Katrina — with dog Princess and her television. "Everybody's talking 'going, going,' but the thing is, when you go, there's no telling what will happen. The storm isn't going to just hit here."
Young, who lives in a new, two-story home built to replace the one destroyed by Katrina, said she wasn't worried about the levees.
"If the wind isn't too rough, I can stay right here," she said, tapping on her wooden living room coffee table. "If the water comes up, I can go upstairs."
While far less powerful than Katrina, Isaac posed similar political challenges, a reminder of how the storm seven years ago became a symbol of government ignorance and ineptitude.
Political fallout was already simmering. Republican Gov. Bobby Jindal, who canceled his trip to the convention, said the Obama administration's disaster declaration fell short of the federal help he had requested, and asked for a promise to be reimbursed for storm preparation costs.
"We learned from past experiences, you can't just wait. You've got to push the federal bureaucracy," Jindal said.
FEMA Administrator Craig Fugate said such requests would be addressed after the storm.
Obama promised that Americans will help each other recover, "no matter what this storm brings."
"When disaster strikes, we're not Democrats or Republicans first, we are Americans first," Obama said at a campaign rally at Iowa State University. "We're one family. We help our neighbors in need."
In Tampa, the storm's landfall did not appear to affect prime-time coverage or the Republican National Convention speeches. One of the few mentions of the storm came in the opening remarks by Ann Romney, wife of the Republican nominee.
"Just so you all know, the hurricane has hit landfall and I think we should take this moment and recognize that fellow Americans are in its path and just hope and pray that all remain safe and no life is lost and no property is lost," she told the crowd.
Outside, though, the streets of downtown Tampa were eerily deserted, a result of nasty weather from Isaac's outer bands, tight securities that blocked off streets and a delay in convention events because of fears the storm might target that side of the Gulf.