The storm swiped south Florida on Sunday before moving into warm Gulf waters, where it is expected to strengthen into a hurricane.

On its current track, Isaac was due to slam into the Gulf Coast anywhere between Florida and Louisiana by Tuesday night or early Wednesday, the seventh anniversary of Katrina hitting New Orleans, the U.S. National Hurricane Center NHC.L said.

"The weather is going to go downhill well in advance of that and that's why today is the day of preparation," said NHC director Richard Knabb.

Speaking in an interview with CNN, Knabb said coastal flooding or storm surge up to 12 feet (3.6 meters) was the biggest threat posed by Isaac, with mandatory evacuations possible across southeastern Louisiana, Mississippi and Alabama.

The governors of all three states have declared states of emergency as a hurricane warning went into effect for the northern Gulf Coast from Louisiana to the Florida Panhandle.

That included New Orleans, devastated when Hurricane Katrina swept over the city on August 29, 2005, killing more than 1,800 people and causing billions of dollars of damage along the coast.

"It is difficult to realize that to the day - seven years after Katrina - another hurricane is headed our way," Mississippi Governor Phil Bryant said.

Late Monday morning, Isaac was about 310 miles (500 km) southeast of the mouth of the Mississippi River with top sustained winds of 65 mph (100 kph) and moving northwest at 14 mph (24 kph).

It was expected to be centered over the Gulf Coast no later than early on Wednesday. Evacuation orders for some low-lying parts of the Gulf Coast already were in effect Monday morning.

Energy producers in the Gulf worked to shut down some of their operations ahead of what could be the biggest test for U.S. energy installations since 2008, when Hurricanes Gustav and Ike disrupted offshore oil output for months and damaged onshore natural gas processing plants, pipelines and some refineries.

Gulf residents started stocking up on supplies and securing their homes. In New Orleans, long lines formed at some gas stations and in Gulfport, Mississippi, people crowded supermarkets to buy bottled water and canned food.

"I sense a high level of anxiety," said New Orleans Mayor Mitch Landrieu. "The timing, as fate would have it, on the anniversary of Katrina has everybody in a state of alertness, but that is a good thing."

Isaac is forecast to become a hurricane on Tuesday. In its latest advisory, the NHC said the storm was not expected to strengthen beyond Category 1, the weakest type on the five-step Saffir-Simpson scale of hurricane intensity.

But Knabb said very warm water temperatures in the Gulf, currently ranging between 86 to 87 degrees Fahrenheit (30-30.5 Celsius) could help trigger strengthening beyond Category 1.

"The Gulf of Mexico is almost always plenty warm in August and September to support strengthening tropical storms and hurricanes," he said.

NHC meteorologist Jessica Schauer said the hurricane warning area included "quite a few oil rigs" but not perhaps the heart of the U.S. offshore oil patch, which produces about 23 percent of U.S. oil output and 7 percent of its natural gas.

Despite the threat to offshore oil infrastructure and Louisiana refineries, U.S. crude oil prices were off in late morning trading after being up earlier in Asia. U.S. oil prices were down about $1.15 at $95.

SHUTTING OIL PRODUCTION

Meteorologists at Weather Insight, an arm of Thomson Reuters, predict the storm will spur short-term shutdowns of 85 percent of the U.S. offshore oil production capacity and 68 percent of the natural gas output.

Once ashore, the storm could wreak havoc on low-lying fuel refineries along the Gulf Coast that account for about 40 percent of U.S. refining capacity.

That could send gasoline prices spiking just ahead of the U.S. Labor Day holiday, analysts said. "It's going right in the heart of refinery row," Phil Flynn, an analyst with Price Futures Group in Chicago, said on Sunday.

London-based BP Plc (BP.L), the biggest U.S. Gulf producer, said it was shutting production at all of its Gulf of Mexico oil and gas platforms and evacuating all workers on Sunday.

Issac's westward track meant the worst of its weather would miss Tampa, Florida, where the Republican National Convention was to open its four-day meeting on Monday. Official convention events were delayed until Tuesday because of the storm.

Tampa, located on Florida's west coast, still faces total rain accumulations of about 15 inches (38 cm) between Sunday and Monday evening, forecasters said.

In south Florida, winds from Isaac forced cancellations of hundreds of flights in and out of Miami, Fort Lauderdale and other south Florida airports on Sunday. Miami-Dade Mayor Carlos Gimenez reported more than 500 cancellations affecting Miami International Airport alone.

The storm killed at least 20 people and caused significant flooding and damage in Haiti and the Dominican Republic before sweeping across the southern tip of Florida on Sunday. (Additional reporting by David Adams and Kevin Gray in Miami, Emily Le Coz in Tupelo, Matthew Robinson in New York and Kristen Hays in Houston; Writing by Tom Brown; Editing by Bill Trott)

• A Permanent Economic Emergency