PENSACOLA -- Strengthening as it loomed just off southeastern Louisiana’s bayous Tuesday, Hurricane Isaac began a slow, flooding crawl that promised to impact much of the Gulf Coast.

Though the storm finally clocked in at hurricane strength in the hours before a portion of its center made landfall in Plaquemines Parish, 90 miles southeast of New Orleans, water was more of a concern than its Category 1 winds. It was on a path to move onshore overnight south of New Orleans, pushing storm surge up to 12 feet that could spill over low-lying coastal Louisiana and Mississippi, followed by up to 20 inches of rain over the next few days as the storm slows.

“When Isaac comes ashore some time tonight, somewhere in Louisiana or Mississippi, that will not be the end,” said National Hurricane Center Director Rick Knabb during a conference call. “It will be the beginning.”

Hurricane warnings were in effect from Morgan City, La., to the Mississippi-Alabama border. The approaching storm forced the shutdown of 49 offshore drilling platforms in the Gulf of Mexico — 65 percent of the operating rigs — and 503 production rigs as well as a sting of refining facilities, reported IHS, a business analysis firm.

For the Florida Panhandle, the threat was downgraded to a tropical storm warning. Florida’s beachfront towns could see eroding sea conditions and flooding was possible across the region, but the sense was that the state had dodged the worst.

At The Fish House, a landmark bayfront eatery in Pensacola, a power surge caused the lights to flicker and the radio to go silent during lunch service Tuesday. It was nothing compared to what happened eight years ago, when Hurricane Ivan ripped the clay shingles from the roof and sent wooden planks from a nearby marina crashing into the building, closing the restaurant for six months.

“I’m relieved, to say the least,” restaurant maintenance manager Robby Quina said Tuesday. “We’re just going to get some feeder bands. Nothing to it.”

Isaac was expected to make landfall a day before — or on — the seventh anniversary of Hurricane Katrina flooding New Orleans. Federal and state disaster managers and political leaders were confident that the city’s beefed up pumps and protective levees would withstand the test of Isaac, a far less intense storm.

“We don’t expect a Katrina-like event, but remember there are things about a Category 1 storm that can kill you,” Mayor Mitch Landrieu said. But there was high concern for residents in low-lying areas outside the levees.

President Barack Obama warned Gulf Coast residents to heed evacuation warnings from local and federal officials. Vulnerable communities in Louisiana, Mississippi, Alabama and Florida had been ordered to pack up in advance of the massive storm.

“Now is not the time to tempt fate. Now is not the time to dismiss official warnings,” Obama said. “You need to take this seriously.”

In South Florida, Isaac left a wake of health advisories and flood warnings.

Health departments in both Miami-Dade and Broward counties urged residents to take extra steps to prevent mosquito bites because the insects, which can carry illness, can breed in the puddles and standing water left behind by the storm.

“West Nile Virus, St. Louis Encephalitis, Eastern Equine Encephalitis and Dengue Fever are known diseases carried by mosquitoes,” said a Broward alert. “Taking appropriate precautions will help to prevent mosquito-borne diseases.”