Updated at 6 p.m. ET: Tropical Storm Isaac was hugging the northern coastline of eastern Cuba on Saturday after claiming at least four lives in Haiti. Isaac should become a Category 1 hurricane on Sunday just as it nears the Florida Keys, the U.S. National Hurricane Center said, and then grow into an even stronger Category 2 storm with 100 mph winds.
Isaac "could be significantly stronger than currently forecast" once it enters the Gulf of Mexico, the center said in an advisory.
It will first sweep past southwest Florida and the Florida Keys, where "hurricane conditions are expected ... Sunday," it said in a separate update.
Isaac is a massive storm, with tropical storm-force winds extending 230 miles from the center. Key West International Airport was halting all flights at 7 p.m. Saturday until the storm had passed.
In Haiti, a woman and a child in the town of Souvenance were killed in the storm, a local official reported. A woman in the southern coastal city of Jacmel was crushed to death when a tree fell on her house, government officials said.
In the capital Port-au-Prince -- where some 350,000 people are still living in tents or shelters after the 2010 Haiti earthquake -- a girl, 10, was killed when a wall fell on her.
Power outages and flooding were reported as Isaac moved across the hilly and severely deforested Caribbean country.
"There's a lot of rain, a lot of wind," said Magdala Jean-Baptiste, who huddled with her frightened children in their home in the southern coastal city of Jacmel. "We haven't had any power since the storm started yesterday. We passed the night with no sleep."
In neighboring Dominican Republic, Isaac felled power and phone lines and left at least a dozen towns cut off by flood waters. The most severe damage was reported along the south coast, including the capital Santo Domingo, where more than half the city was without power.
Cuba prepared by closing beaches and evacuating tourists in vulnerable areas, NBC's Mary Murray and The Weather Channel's Mike Seidel reported from Havana. Flights across Cuba were also suspended.
In Baracoa, a city on Cuba's eastern side, high seas began topping the seawall Friday night, Radio Baracoa reported.
Now with 60-mph winds, Isaac should exit Cuba on Sunday and then move south of the Florida Keys and into the Gulf.
Florida Gov. Rick Scott on Saturday declared a state of emergency to make sure local and state agencies would be ready. Republicans effectively canceled the first day of their national convention in Tampa, on Florida's central Gulf Coast, deciding to gavel it open on Monday, then immediately recess to some time on Tuesday.
Gulf of Mexico operators began shutting down offshore oil and gas rigs on Friday ahead of the storm.
Tampa's weather forecast includes rain and high winds Sunday night and into Monday, The Weather Channel reported. The winds could gust up to 60 mph.
Monday and Tuesday include a risk of tornadoes across south Florida.
Officials were handing out sandbags to residents in the Tampa area, which often floods when heavy rainstorms hit. Sandbags also were being handed out in Homestead, 20 years after Hurricane Andrew devastated the community there. Otherwise, however, convention preparations were moving ahead as usual.
Isaac's exact path is still unclear, but the hurricane center said models suggest it will make landfall somewhere between the Florida Panhandle and New Orleans on Tuesday night.
The storm's anticipated path did shift closer to the Keys than previously forecast and emergency managers urged tourists to leave the islands if they could do so safely. A single road links the chain of islands to the Florida Peninsula.
Reuters and The Associated Press contributed to this report.
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