Louisiana Gov. Bobby Jindal called a state of emergency and suggested that people begin leaving low-lying parts of the state. He also said he may skip a speaking engagement later this week at the Republican National Convention in Tampa if the storm is threatening his state.
The storm was on a course to pass west of Tampa, but it had already disrupted the Republicans' schedule there because of the likelihood of heavy rain and strong winds that extended more than 200 miles from its center.
Even before reaching hurricane strength, Isaac caused considerable inconvenience, with more than 550 flights canceled at Miami International Airport and about 150 from Fort Lauderdale's airport. There were scattered power outages from Key West to Fort Lauderdale affecting more than 8,000 customers, and flooding occurred in low-lying areas. No serious damage was reported.
Wind gusts of 60 mph were reported as far north as Pompano Beach, north of Fort Lauderdale. But while officials urged residents in southeast Florida to stay home, that recommendation was ignored by surfers and joggers on Miami Beach and shoppers at area malls.
In Key West, Emalyn Mercer rode her bike while decked out with a snorkel and mask, inflatable arm bands and a paddle, just for a laugh. She rode with Kelly Friend, who wore a wet suit, dive cap and lobster gloves.
"We're just going for a drink," Mercer said.
"With the ones that are brave enough like us," Friend added.
Along famed Duval Street, many stores, bars and restaurants closed, the cigar rollers and palm readers packed up, and just a handful of drinking holes remained open.
But people posed for pictures at the Southernmost Point, while at a marina Dave Harris and Robyn Roth took her dachshund for a walk and checked out boats rocking along the waterfront.
"Just a summer day in Key West," Harris said.
That kind of ho-hum attitude extended farther up the coast. Edwin Reeder swung by a gas station in Miami Shores not for fuel, but drinks and snacks.
"This isn't a storm," he said. "It's a rain storm."
With a laugh, Reeder said he has not stocked up aside from buying dog and cat food.
The forecast wasn't funny, however. Isaac was expected to draw significant strength from the warm, open waters of the Gulf of Mexico, but there remained much uncertainty about its path.
The Gulf Coast hasn't been hit by a hurricane since 2008, when Dolly, Ike and Gustav all struck the region.
Hurricane center forecasters are uncertain of the storm's path because two of their best computer models now track the storm on opposite sides of a broad cone. One model has Isaac going well west and the other well east. For the moment, the predicted track goes up the middle.
Florida Panhandle residents stocked up on water and gasoline, and at least one Pensacola store ran out of flashlight models and C and D batteries. Scott Reynolds, who lives near the water in Gulf Breeze, filled his car trunk with several cases of water, dozens of power bars and ramen noodles.
"Cigarettes I'm stocking up on those too," he said.
Forecasters stressed that the storm's exact location remained extremely uncertain a fact not lost on Tony Varnado as he cut sheets of plywood to board up his family's beach home on Pensacola Beach. With the storm's projected path creeping farther to the west, the Mandeville, La., resident joked he might be boarding up the wrong house.
"I'm going to head back that way as soon as we are done here to make sure we are prepared if hits there," he said.
Before reaching Florida, Isaac was blamed for seven deaths in Haiti and two more in the Dominican Republic, and downed trees and power lines in Cuba. It bore down on the Keys two days after the 20th anniversary of Hurricane Andrew, which caused more than $25 billion in damage just north of the island chain.
In Tampa, convention officials said they would convene briefly on Monday, then recess until Tuesday afternoon, when the storm was expected to have passed. Gov. Rick Scott canceled his plans to attend convention events on Sunday and Monday.
At Miami International Airport, more than 550 flights Sunday were canceled. Inside the American Airlines terminal, people craned for a look out of one of the doors as a particularly strong band of Isaac began lashing the airport with strong rain and high wind.
Michele Remillard said she was trying to get a seat on a flight to New Orleans, well aware the city could be affected by Isaac later this week. In coastal Plaquemines Parish, La., crews rushed to protect the levees that keep floodwaters from reaching that New Orleans suburb.
"It's a little scary," said Remillard, who was in town for a wedding. "But I need to get home, you know? And if the storm comes my way again, who knows, I might have to come back here."
In Mississippi, officials were drafting an emergency declaration that the governor could sign as early as Sunday. Evacuations had not yet been ordered but were likely, especially in areas vulnerable to storm surge, said Greg Flynn, spokesman for the Mississippi Emergency Management Agency.
As of 5 p.m. EDT, the storm was centered about 40 miles (65 kilometers) south of Key West, according to the National Hurricane Center in Miami. Isaac had top sustained winds of 60 mph (95 kph) and was moving to the northwest at 16 mph (26 kph).
Tropical storm-force winds extended outward up to 205 miles (335 km) from the center, meaning storm conditions are possible even in places not in Isaac's direct path.
Associated Press writers Tony Winton in Key West, Melissa Nelson in Pensacola, Fla., and Tim Reynolds, Curt Anderson and Suzette Laboy in Miami contributed to this report.