Isaac's winds, rain and storm surge could pose a major test of New Orleans' new flood control systems and reinforced levees. Forecasts from the U.S. National Hurricane Center showed the storm coming ashore late on Tuesday.

Energy companies evacuated offshore oil rigs and shut down U.S. Gulf Coast refineries as the storm threatened to batter the oil refining belt. As markets mulled Isaac's potential to tighten fuel supplies, prices for international benchmark Brent crude were up 46 cents to $112.72 a barrel in early Tuesday activity.

"Isaac is expected to become a hurricane before making landfall," the hurricane center said in its 11 a.m. EDT (1500 GMT) advisory. "The center of Isaac should reach the coastline of southeastern Louisiana as early as this evening," it said.

Isaac was centered 80 miles (125 km) southeast of New Orleans and the mouth of the Mississippi River with top sustained winds of 70 miles per hour (110 kph), just short of the 74 mph (119 kph) hurricane status.

Its forward speed was a relatively slow 10 mph (17 kph), a red flag for hurricane and tropical storm experts since slow-moving cyclones can bring higher rainfall totals. The storm was about 370 miles (595 km) wide.

Isaac spared Tampa, Florida, where the Republican National Convention began on Monday. But it forced party leaders to revamp their schedule and they may have to make further revisions so as not to be seen celebrating Mitt Romney getting the party's presidential nomination while Gulf Coast residents are struggling through the storm.

OBAMA ADDS WARNING

President Barack Obama added his concerns in a statement from the White House, saying, "We're dealing with a big storm and there could be significant flooding and other damage across a large area.

"Now is not the time to tempt fate," he added, saying people should heed warnings and evacuate if instructed by authorities to do so.

Rain and tropical storm force winds were expected to spread into the Gulf Coast region in the coming hours, bringing the threat of storm surge and flooding.

Isaac had New Orleans in its sights as the city still struggles to recover from Hurricane Katrina, which swept across it on August 29, 2005, killing more than 1,800 people and causing billions of dollars of damage.

Authorities have urged thousands of residents in low-lying areas to evacuate since Monday, warning the storm could flood towns and cities in at least three U.S. Gulf Coast states with a storm surge of up to 12 feet (3.6 meters).

Rainfall accumulations, potentially totaling as much as 20 inches (50 cm) in some areas, could also trigger widespread flooding.

"Inland flooding historically has been responsible for a lot of fatalities in tropical storms and hurricanes coming ashore," Hurricane Center director Rick Knabb told CNN. "It's not just going to be a coastal event."

Residents in coastal communities from Louisiana to Mississippi stocked up on food and water and tried to secure their homes, cars and boats.

Isaac was forecast to strengthen into a Category 1 hurricane, the lowest on the Saffir-Simpson scale, with top winds of 85 mph (137 kph). While that would be well below the intensity of Katrina, which was a Category 3 storm, the size of Isaac's slow-moving system has forecasters predicting widespread flooding.

'RIDING IT OUT'

As the sun rose Tuesday in New Orleans, gusts of wind were the only signs of an approaching storm but along Canal Street in the historic French Quarter, crews were boarding up the windows of stores and businesses.

"I'm not all that concerned about the storm. It's still a Category 1," said Charles Neeley, a 69-year-old contractor overseeing workers covering the windows of a CVS drugstore. "We usually ride out ones and twos and get the hell out for threes and fours."

Nonetheless, Neeley said he had stocked up on food and water at home and fuel for his generator.

Karen Foley, a 23-year-old tourist had planned to travel home to New Jersey with a friend.

"Our flights were canceled so we're going to be here," she said. "We are just hoping the city doesn't get hit again. It doesn't deserve it."

After Katrina, the U.S. Army Corps of Engineers built a $14.5 billion defense flood system of walls, floodgates, levees and pumps designed to protect the city against a massive tidal surge like the one that swamped New Orleans in Katrina's wake.

In low-lying Plaquemines Parish, which could be the first to be lashed by Isaac's winds and storm surge, workers scrambled to stack sandbags and reinforce levees as Isaac lurked in the Gulf.

Much of the parish lies outside the greater New Orleans levee system, and construction projects to bolster protection are not yet complete.

"We're really worried about the storm surge. We really need a few more years before we see an event like this," said Plaquemines Parish President Billy Nungesser.

As a precaution, as of Monday afternoon, the energy industry had shut down 78 percent of U.S. Gulf of Mexico crude production and 48 percent of its natural gas production, government figures showed .

The U.S. offshore oil patch produces about 23 percent of the country's oil output and 7 percent of its natural gas.

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.

The ports of Mobile, Alabama, and New Orleans were closed and barge traffic was suspended along southern portions of the Mississippi River.

Isaac killed at least 22 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday. (Additional reporting by Tom Brown, Jane Sutton, David Adams and Kevin Gray in Miami, Ben Gruber and Kathy Finn in New Orleans, Emily Le Coz in Tupelo, Missisippi, Kristen Hays, Erwin Seba and Chris Baltimore in Houston and Verna Gates in Alabama; Writing by Tom Brown; Editing by Bill Trott)

• A Permanent Economic Emergency