NEW ORLEANS (Reuters) - Tropical Storm Isaac closed in on the U.S. Gulf of Mexico coast on Monday, triggering some mandatory evacuation orders and disrupting U.S. offshore oil production as it threatened to make landfall between Florida and Louisiana as a Category 2 hurricane.
The wide, slow-moving storm swiped south Florida on Sunday and strengthened over the warm Gulf waters. It was expected to reach land Tuesday night or early Wednesday, the anniversary of devastating Hurricane Katrina seven years ago.
The U.S. National Hurricane Center warned that the storm could push vast amounts of seawater over the shore, flooding the northern Gulf coast with a storm surge of up to 12 feet in some areas. Isaac was expected to slow down and pour "tremendous amounts" of rain on the region, causing potentially deadly flooding far inland, Hurricane Center Director Rick Knabb said.
"The weather will start going downhill overnight tonight on the northern Gulf Coast," Knabb told reporters on a conference call. "Wherever it is people are going to be during the storm, they need to get there tonight."
Isaac could take direct aim at New Orleans, which is still struggling to fully recover from Katrina which swept across the city on August 29, 2005, killing more than 1,800 people and causing billions of dollars of damage along the coast.
"That brings a high level of anxiety to the people of New Orleans," New Orleans Mayor Mitch Landrieu told a news conference. "I want to tell everybody now that I believe that we will be OK," he added.
At 5 p.m. EDT (2100 GMT) on Monday, Isaac was centered 255 miles southeast of the mouth of the Mississippi River with top sustained winds of 70 mph and swirling northwest at 12 mph.
It was forecast to strengthen into a Category 2 hurricane on the five-step Saffir-Simpson scale of hurricane intensity, with top winds of 100 mph, before moving over the Gulf Coast no later than early on Wednesday. A tropical storm becomes a Category 1 hurricane at 74 mph.
"Strengthening is expected to continue right up until landfall occurs," the National Hurricane Center said.
The storm was more than 400 miles wide and Craig Fugate, director of the Federal Emergency Management Agency, said the worst effects may well be in Mississippi and Alabama.
"This is not a New Orleans storm. This is a Gulf Coast storm," Fugate said.
The governors of Louisiana, Mississippi and Alabama declared states of emergency on Sunday and mandatory evacuation orders went into effect on Monday for residents of several low-lying districts outside New Orleans and its new flood-protection system. About 8,000 to 10,000 residents of Mobile, Alabama, were also told to evacuate, as well as nearby Dauphin Island.
Energy producers in the Gulf 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.
The ports of Mobile and New Orleans were closed on Monday and barge traffic was suspended along southern portions of the Mississippi River.
President Barack Obama on Monday approved Louisiana's request for a federal disaster declaration, Governor Bobby Jindal said. Obama's approval, given in a phone call that also included governors of Mississippi, Alabama and Federal Emergency Management Agency Administrator Craig Fugate, makes federal funds available for disaster recovery activities like clearing debris.
In New Orleans, which sits below sea level, long lines formed at some gas stations. In Gulfport, Mississippi, as in many other coastal towns, people crowded supermarkets to buy bottled water and canned food.
A bumper-to-bumper stream of vehicles left New Orleans on the I-10 highway heading west toward Baton Rouge on Monday as motorists made their way to higher ground.
At Mandina's Restaurant, a popular New Orleans eatery flooded by eight feet of water during Katrina, fourth-generation owner Cindy Mandina said she was nervous.
"We're going to hold tight and hope for the best," Mandina said, as she prepared to close up ahead of the storm. "Pre-Katrina, you'd never close, you'd stay open, maybe lose power and then reopen as soon as possible."
At the Crescent City Auction Gallery, owner Adam Wolf Lambert was putting metal shutters up over his store's large ground-level windows on Monday afternoon.
"We're putting up the shutters and moving as much as we can off the ground floor, to be sure we don't have water damage," Lambert said.
Colonel Edward Fleming, commander of the U.S. Army Corps of Engineers overseeing New Orleans flood protection, said improvements to the system put the city in a far better place than it was seven years ago.
But 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.
The parish, which stretches southeast from New Orleans, is cut in two lengthwise by the Mississippi River as it flows to the Gulf of Mexico. Much of the area lies outside the greater New Orleans levee system, and construction projects to bolster protection are not yet complete.
"We signed an agreement with the (Army Corps of Engineers) 30 days ago for over a billion dollars of work," said Plaquemines Parish President Billy Nungesser. "We're really worried about the storm surge, we really need a few more years before we see an event like this."
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, prices for international benchmark Brent crude traded down $1.24 to $112.35 a barrel in late Monday activity.
Meteorologists at Weather Insight, an arm of Thomson Reuters, estimated the storm temporarily shut down 87 percent of the U.S. offshore oil production capacity and 80 percent of the offshore 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 on September 3, analysts said.
Isaac's projected track meant the worst of its weather would miss Tampa, Florida, where the Republican National Convention opened its four-day meeting on Monday. Official convention events were delayed until Tuesday because of the storm.
Isaac killed at least 20 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 Jane Sutton and David Adams in Miami, Scott Malone and Ben Gruber in New Orleans, Emily Le Coz in Tupelo, Kristen Hays and Chris Baltimore in Houston and Verna Gates in Alabama; Writing by Tom Brown; Editing by Jackie Frank