U.S. President Barack Obama has declared a state of emergency in Louisiana as the Gulf Coast state prepares for tropical storm Isaac.

HURRICANES Graphic: Inside the eye of a tropical storm Anatomy of a cyclone

The declaration makes federal funding available for emergency activities related to the storm, and followed a phone call on Monday between Obama and Louisiana Governor Bobby Jindal, the White House said.

Obama has asked the Federal Emergency Management Agency to co-ordinate disaster relief efforts with state and local officials along the Gulf Coast.

Isaac is expected to be a Category 2 hurricane with sustained winds of 161 km/h by the time it reaches land on Tuesday night or early Wednesday, according to the latest forecast from the National Hurricane Center in Miami. On Monday evening it was over the Gulf of Mexico — gaining strength over the warm, open waters and moving northwest at about 19 km/h with sustained winds of 112 km/h.

Some people braved the rain, including this person at Clarence Higgs Beach in Key West, Fla., as tropical storm Isaac hit the area on Sunday.Some people braved the rain, including this person at Clarence Higgs Beach in Key West, Fla., as tropical storm Isaac hit the area on Sunday. (Walter Michot/The Miami Herald/Associated Press)

The storm is expected to make landfall between Louisiana and Florida — somewhere along a roughly 500-kilometre stretch from the bayous southwest of New Orleans to the Florida Panhandle — arriving one day shy of seven years after Hurricane Katrina struck in 2005.

Earlier Isaac left 24 dead in Haiti blew past the Florida Keys with little damage and promised a drenching but little more for Tampa, where the planned Monday start of the Republican National Convention was pushed back a day in case Isaac passed closer to the bayside city.

Florida feels Isaac

In the Florida Keys, Monroe County Sheriff's spokeswoman Becky Herrin said there were no injuries and few reports of damage as the storm crossed near Key West on Sunday.

Forecasters said Isaac could pack a double punch of flood threats for the Gulf Coast. If it hits during high tide, the storm could push floodwaters as deep as four metres on shore in Louisiana, Mississippi and Alabama and up to two metres in the Florida Panhandle.

Republicans consider shorter convention

Tampa, the Republican National Convention's host city, has been spared any severe weather from Isaac, though organizers postponed or cancelled most planned first-day events for Monday over safety concerns.

But Isaac's path toward the U.S. Gulf Coast has reportedly forced organizers to consider further amendments to the schedule.

A report from the National Journal suggests Republican officials have not ruled out different scenarios, including a quick roll-call vote and only presidential nominee Mitt Romney speaking at the event.

Meanwhile, the Boston Globe, citing unnamed Romney campaign advisers, is reporting there is a "distinct possibility" the convention will be cancelled altogether.

Much of the rest of South Florida remained under a tornado watch early Monday as the remnants of Isaac moved across the area.

"Our biggest problem right now in the state is our concern about the panhandle," said Florida Governor Rick Scott at a press conference Monday afternoon. "We're going to start seeing tropical storm force winds tonight."

The northwestern region of Florida can expect a storm surge of 76-152 mm, and a rainfall of 381 mm, Scott said.

In Tampa, flooding prompted the closure of several main roads early Monday including three near downtown where the Republican convention was to be held.

Louisiana Governor Bobby Jindal called a state of emergency, and 53,000 residents of St. Charles Parish near New Orleans were told to leave ahead of the storm.

Mississippi Governor Phil Bryant and Alabama Governor Robert Bentley also declared states of emergency.

Meanwhile, the oncoming storm stopped work on rigs that account for 24 per cent of daily oil production in the U.S. potion of the Gulf of Mexico and eight percent of daily natural gas production there, the federal Bureau of Safety and Environmental Enforcement said in its latest update Sunday.

Several area governors have altered their plans for this week's GOP convention in Tampa. Bentley has canceled his trip, and Jindal said he's likely to do so unless the threat from the storm subsides. Florida Governor Rick Scott gave up a chance to speak.

Hurricane warnings

In addition to hurricane warnings in four states from Louisiana to the Florida Panhandle, tropical storm warnings were effect for a section of Louisiana's Gulf Coast from Morgan City to Intracoastal City. Tropical storm warnings were also in effect for many areas along Florida's Gulf Coast.

Amtrak cancelled train service in Louisiana for Tuesday and Wednesday. The route than runs from New York to New Orleans would end in Atlanta, while its route from Los Angeles to New Orleans would stop in San Antonio. Amtrak was also suspending part of its rail line between Miami and Orlando, Fla.

Grocery and home improvement stores as well as fuel stations in Louisiana reported brisk business as residents sought to prepare for Isaac. Some gas stations were running out of supplies.

Even though the storm was moving well west of Tampa, tropical storm-force winds and heavy rains were possible in the area because of Isaac's large size, forecasters said.

A small group of protesters braved rainy weather Sunday and vowed to continue despite the weather, which already forced the Republicans to cancel Monday's opening session of the convention. Instead, the GOP will briefly gavel the gathering to order Monday afternoon and then recess until Tuesday.

Tampa Mayor Bill Buckhorn, a Democrat, said the weather would be "squirrely," but predicted the storm would not unduly interfere with the convention.

"We're going to show the world on Tuesday, Wednesday and Thursday what a great place this is," he said. "As a state and a city, we're going to put on a good show and be a great host for the Republican Party."