Isaac is lashing coastal areas with winds of up to 80mph (130km/h) and it is expected to reach New Orleans exactly seven years after it was hit by Hurricane Katrina.
The city has closed the new floodgates designed to protect it from flooding.
President Barack Obama has warned residents in Isaac's path not to "tempt fate" and to heed evacuation warnings.
Thousands of people have fled the area; those who have stayed behind have locked themselves indoors.

The storm is estimated to be located about 90 miles (145km) south-east of New Orleans and is moving at about 8ph (13km/h), the NHC said.
The BBC's Alastair Leithead in New Orleans says roadworks are being pulled apart by winds and trees are being shaken from their roots.
Mr Obama has declared an emergency in Louisiana and Mississippi, allowing federal funds to be released to local authorities.
"I want to encourage all residents of the Gulf Coast to listen to your local officials and follow their directions - including if they tell you to evacuate," Mr Obama said on Tuesday.
Speaking from the White House, he added: "Now is not the time to tempt fate. Now is not the time to dismiss official warnings. You need to take this seriously."
'Great wall' of New Orleans Shortly before Isaac reached hurricane status on Tuesday, Louisiana Governor Bobby Jindal called for the Federal Emergency Management Agency (Fema) to make a full emergency declaration for the state.
He told reporters that a previous declaration did not allow for the reimbursement for state's expenses from the storm.
"We have learned from past experiences that you cannot wait and you have to push the federal bureaucracy," Mr Jindal, who cancelled an appearance at the Republican National Convention because of the storm, said.
New Orleans Mayor Mitch Landrieu said his city was "officially in the fight" on Tuesday, as he confirmed its airport was closed and would not serve as a shelter.



Mr Landrieu said that a 26ft (8m) high levee gate that now protects the areas of the city that were badly flooded in 2005 - which he dubbed "the great wall of New Orleans" - was closed on Tuesday morning.





"We will not have a Katrina-like event," he said, adding there will still be parts of the city that will likely be flooded.
"Do not let this storm lull you into complacency," he said. "People may be getting bored. It's better to be bored than to get hurt."
Officials have not ordered any evacuations, telling residents to reinforce their homes and stock up on supplies instead.