Forecasters say Isaac has strengthened into a Category 1 hurricane with 75 mph winds as it makes its way toward Louisiana.
The U.S. National Hurricane Center said Tuesday that Isaac gained strength as it moved over the warm, open waters of the Gulf of Mexico.
Isaac is expected to make landfall over southeastern Louisiana, possibly the New Orleans area, either late Tuesday or early Wednesday. Wednesday is the seventh anniversary of the day the Crescent City was devastated by Hurricane Katrina.
Rain and tropical storm force winds were expected to spread into the region in the coming hours, bringing the threat of storm surge and flooding.
President Barack Obama warned residents Tuesday that there could be significant flooding and encouraged people to evacuate if instructed by authorities to do so.
"As we prepare for Isaac to hit, 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," Obama said at the White House before departing on a two-day campaign trip.
"Now is not the time to tempt fate, now is not the time to dismiss official warnings. You need to take this seriously," he said.
Obama's statement was meant to show he is taking the storm seriously, too, despite his campaign travel.
Isaac is expected to make landfall in the New Orleans area seven years after it was devastated by Hurricane Katrina. Obama has criticized the Bush administration's handling of the Katrina disaster
Isaac had New Orleans in its sights as the city still struggles to recover from Hurricane Katrina which swept across it almost exactly 7 years ago on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage.
Authorities urged thousands of residents in low-lying areas to evacuate, 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.
Isaac also threatened heavy rainfall, with possibly as much as 18 inches in areas, potentially triggering flooding in some coastal areas.
Residents in coastal communities from Louisiana to Mississippi stocked up on food and water and tried to secure their homes, cars and boats.
In New Orleans, a bumper-to-bumper stream of vehicles left the city on a highway toward Baton Rouge in search of higher ground. Others prepared, or were forced, to ride the storm out.
Aa Category 1 hurricane ia the lowest on the Saffir-Simpson scale, with top winds of 90 miles per hour. While that would be well below the intensity of Katrina, a Category 3 storm, the size of Isaac's slow-moving system has forecasters predicting widespread flooding.
"Even if it is a tropical storm at landfall, the large size of it will still generate significant storm surge," Hurricane Center Director Rick Knabb told reporters. "That is life-threatening potentially."
Along Canal Street in New Orleans' 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.
"Our flights were canceled so we're going to be here," said Karen Foley, a 23-year-old tourist who had planned to travel home to New Jersey with a friend. "We are just hoping the city doesn't get hit again. It doesn't deserve it."
Isaac was centered 125 miles southeast of the mouth of the Mississippi River with top sustained winds of 70 miles per hour — a speed that places the storm very near hurricane status — and swirling northwest at 12 mph.
The storm was more than 400 miles wide. Craig Fugate, director of the Federal Emergency Management Agency, said the worst effects may well be in Mississippi and Alabama.
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.
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.
Energy companies evacuated offshore oil rigs and shut down U.S. Gulf Coast refineries as the storm threatened to batter the oil refining belt.
Oil firms ferried workers in helicopters from oil platforms hundreds of miles offshore in the Gulf of Mexico.
As a precaution, as of Monday afternoon, the energy industry had shut down 78 percent of Gulf of Mexico crude production and 48 percent of its natural gas production, government figures showed.
Despite the threat to the U.S. offshore oil patch, which produces about 23 percent of the country's oil output and 7 percent of its natural gas, prices for international benchmark Brent crude traded down $1.24 to $112.35 a barrel in late Monday activity.
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.
Louisiana Governor Bobby Jindal said President Barack Obama approved his state's request for a federal disaster declaration, making federal funds available for disaster recovery activities like clearing debris.
The ports of Mobile and New Orleans were closed and barge traffic was suspended along southern portions of the Mississippi River.
See: NOAA's Tracking Map for Isaac
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