As Tropical Storm Isaac moves over the island, Todd Kjeer and Penny Garbus (R) deliberately stand in the path of breaking waves in Key West August 26, 2012.[Agencies]

Isaac was becoming better organized and faced favorable conditions, increasing the possibility that the storm could strengthen beyond a Category 2 hurricane, said NHC meteorologist David Zelinsky.

Tropical-force winds from the massive storm stretched across 400 miles (644 km), with rain bands extending even further, he added.

"It certainly is a large storm," he said, noting that wind gusts of 60 mph (100 kph) 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 after many visitors heeded official warnings to head home early. Isaac began moving away from the Florida Keys on Sunday evening.

Republican convention schedule disrupted   

Republicans, who will formally nominate former Massachusetts Governor Mitt Romney as their presidential candidate for the November election, will convene their four-day meeting on Monday as scheduled, then recess until Tuesday.

Tampa, located on Florida's west coast, still faces a threat of both winds and heavy rains from Isaac. But forecasters said a slight westward shift in the storm's track helped put Tampa out of harm's way.

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 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, which got off lightly when the storm crossed its eastern flank instead of raking up the length of the island as originally predicted.