KEY WEST, Fla - Tropical Storm Isaac lashed south Florida with winds and heavy rain on Sunday after battering the Caribbean, threatening to interrupt most US offshore oil production in the Gulf of Mexico and disrupting plans for the Republican National Convention in Tampa.

Isaac is expected to strengthen to a Category 2 hurricane and hit the Gulf Coast somewhere between Florida and Louisiana at midweek - on or near the seventh anniversary of Hurricane Katrina - the US National Hurricane Center (NHC) said in an advisory.

A hurricane warning was issued for the Gulf Coast from Louisiana to the Florida Panhandle, including New Orleans, which was devastated when Katrina struck the city on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage to the Gulf Coast.

In Louisiana, Governor Bobby Jindal declared a state of emergency and said 15 low-lying parishes outside New Orleans' newly built, $14.5 billion flood defense system would likely be under mandatory evacuation orders by Monday.

"There's really nothing that's going to stop this storm from forming and from strengthening," said Jindal, a seasoned crisis manager who has weathered such disasters as the 2010 BP oil spill.

On Mississippi's Gulf Coast, residents started stocking up on supplies and securing their homes. "It is packed. They are clearing the shelves," said Lindy Stewart after shopping at a Sam's Club in Gulfport. Stewart said she bought bread, lunch meat and other "stuff you need to survive a couple of days without power."

The Mississippi State Port Authority ordered the port of Gulfport cleared of all cargo vessels and cargo containers.

Isaac is forecast to become a hurricane either late Monday or Tuesday. The NHC said the storm was expected to eventually intensify to a Category 2 hurricane with "extremely dangerous" sustained winds of 105 miles per hour (169 kph) as it swept through the warm waters of the Gulf of Mexico.

Forecasters are predicting a more westward track that could bring Isaac over the heart of the US offshore oil patch, which produces about 23 percent of US oil output and 7 percent of its natural gas output.

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 US offshore oil production capacity and 68 percent of the natural gas output.  

With the threat to offshore oil infrastructure and Louisiana refineries, US crude oil prices traded up 75 cents to $96.90 a barrel in Asia early Monday.

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

That could send gasoline prices spiking just ahead of the US 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 , the biggest US Gulf producer, said it was shutting production at all of its Gulf of Mexico oil and gas platforms and evacuating all workers on Sunday.

At 8 pm EDT (2400 GMT) on Sunday, Isaac was about 60 miles (100 km) southwest of Key West at the southernmost tip of the US mainland, packing top sustained winds of 65 mph (100 kph) and churning west-northwest at 15 mph (24 kph).