The US National Hurricane Center tonight issued a hurricane warning for the northern Gulf of Mexico coast from Louisiana to the Florida Panhandle.
Isaac is still a tropical storm with 100kph winds moving past the Florida Keys and headed into the warm waters of the Gulf of Mexico, the NHC said in its 5pm (10pm Irish time) advisory.
It is forecast to intensify to a Category 2 hurricane by the time it reaches the northern Gulf coast by early Wednesday.
Wednesday is the seventh anniversary of hurricane Katrina, which struck New Orleans in 2005, killing more than 1,800 people and causing billions of dollars of damage.
Isaac lashed south Florida with winds and heavy rain today after battering the Caribbean.
A storm becomes a hurricane when sustained winds reach a minimum of 119kph.
The NHC said Isaac was expected to intensify to a Category 2 hurricane, with "extremely dangerous" sustained winds of 169kph, as it swept up the warm waters of the Gulf of Mexico.
Tropical force winds from the massive storm stretched across 644km, with rain bands extending even further, said NHC meteorologist David Zelinsky.
"It certainly is a large storm," he said, noting that wind gusts of 100kph had been detected as far apart as Key West and Palm Beach.
The winds forced cancellations of hundreds of flights in and out of Miami, Fort Lauderdale and other south Florida airports.
Miami Mayor Carlos Gimenez reported more than 500 cancellations affecting Miami International Airport alone.
More than half of the restaurants and other businesses were shuttered in the tourist haven of Key West today, at the southernmost tip of the US mainland, after many visitors heeded official warnings to head home early.
Republicans, who will formally nominate former Massachusetts Governor Mitt Romney as their presidential candidate for the November election, will convene their four-day meeting tomorrow as scheduled, and then recess until Tuesday.
Eight people killed in Haiti
In Haiti, Isaac added to the misery of more than 350,000 survivors of the 2010 earthquake still living in flimsy resettlement camps as water gushed into tents and corrugated plastic shacks ripped apart by the wind.
Authorities in the impoverished nation said today the storm had killed eight people, including three children.
In the Dominican Republic, officials said three people were missing, including the mayor of a town near Santo Domingo who was swept away as he tried to save another person from a flooded river.
No deaths or injuries were reported in Cuba, where the storm crossed the country’s eastern flank instead of travelling up the length of the island as originally predicted, but it still sustained damage.
