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.

Republicans effectively cancel first day of convention

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.

Dieu Nalio Chery / AP

Residents wade through a flooded street in Port-au-Prince, Haiti, on Saturday.

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. 

Follow Isaac's path with our storm tracker
Live updates and analysis from weather.com

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.

Slideshow: Isaac tracks toward Florida

Walter Michot / AP

Tropical Storm Isaac rakes the Dominican Republic, Haiti and Cuba as it makes its way toward Florida, where Tampa will be hosting the Republican National Convention.

Launch slideshow

More world stories from NBC News:

• Dozens killed, hurt in Venezuela oil refinery explosion
• Tiger at German zoo kills staffer, is shot dead
• Canadian woman in water-soaked wedding dress drowns
• 'Crushing political dissent'? Gambia to execute every prisoner on death row

Follow World News from NBCNews.com on Twitter and Facebook

Scott Iskowitz / AP

Republican National Committee Chairman Reince Priebus, left, and convention CEO William Harris unveil the stage and podium for the 2012 Republican National Convention on Monday in Tampa, Fla.

A federal law enforcement bulletin "assesses with high confidence" that extremists from anti-government anarchist groups will try to use violence with the aim of disrupting both political conventions.

That assessment, it says, is based on what has happened at similar high-profile events.

The analysis, sent this week to local police organizations nationwide by the FBI and Department of Homeland Security, says these groups "probably lack the capability to overcome the heightened security measures" around actual convention facilities but could target businesses or means of transportation nearby. Most of what these groups do, the bulletin says, involves vandalism, trespassing and attempts to block roads. But it says they're becoming more interested in trying to use explosives.

The Republican national convention is scheduled to run Monday through next Thursday in Tampa, Fla. The assessment says the FBI picked up information in mid-March indicating that some anarchists from New York were planning to go to Tampa in hopes of closing "all the Tampa Bay-area bridges" during the convention, although it doesn't say how they planned to do it.

Stay informed with the latest headlines; sign up for our newsletter

Though the bulletin makes no mention of Tropical Storm Isaac, it says that "inclement weather and effective crowd control" could deter the actions of these groups.

Isaac was in the Caribbean Sea on Thursday and expected to pass south of Puerto Rico. Some computer models showed Isaac moving parallel to Florida's western coastline, passing Tampa on Monday. 

Pete Williams is NBC News' chief justice correspondent.

More content from NBCNews.com:

• Embattled tanning industry fights back, taking cues from Big Tobacco
• California school district sued over abstinence-only sex ed
• Video: Owner reunited with ring found 1,000 miles away
• 2 Good Samaritans electrocuted trying to help car crash victim
• Activist state election officers lead charge for Voter ID

Follow US News from NBCNews.com on Twitter and Facebook

Florida Gov. Rick Scott has shot down a request by Tampa's mayor to allow local authorities to ban guns from the city's downtown during the Republican National Convention in August.

Citing Second Amendment protections in the U.S. Constitution, Scott told Tampa Mayor Bob Buckhorn that conventions and guns have co-existed since the nation's birth and would continue to do so during the four-day event beginning Aug. 27.

"It is unclear how disarming law abiding citizens would better protect them from the dangers and threats posed by those who would flout the law," the Republican governor said in a letter on Tuesday.

Local officials need Scott's permission to enact the temporary restrictions after state lawmakers last year passed a measure that prohibits local governments from adopting gun ordinances that are stricter than state law.

Florida has some of the most lenient gun laws in the United States and by some counts leads the nation in gun ownership, with about 6.5 percent of all adults licensed to carry a concealed weapon, state records show.

New applications for concealed gun permits have quadrupled since 1998.

In a letter to Scott, Buckhorn said the Tampa City Council had banned a host of items from the area surrounding the convention facility, a list that includes water guns, poles and pieces of wood.

"One noticeable item missing from the city's temporary ordinance is firearms," Buckhorn wrote. "In the potentially contentious environment surrounding the RNC, a firearm unnecessarily increases the threat of imminent harm and injury to the residents and visitors to the city."

Scott said he was confident law enforcement officials, who are expected to number nearly 4,000, would be able to protect the public without having to enforce a blanket gun ban.

That city officials have banned other items is irrelevant, he said.

Watch the most-viewed videos on msnbc.com

"The choice to allow the government to ban sticks, poles but not firearms, is one that the people made in enacting their state and federal constitutions," Scott wrote. 

Weapons will not be allowed in the convention center itself or in the immediate area surrounding the site. Security in that venue is being handled by the U.S. Secret Service. 

The City Council wants to extend the restrictions to all of downtown, including areas that have been designated zones for protesters expected at the event. 

"As governor, you have the duty to meet dangers presented by events such as the RNC where there is a threat of substantial injury and harm to Florida residents and visitors to the state," Buckhorn wrote.

More content from msnbc.com and NBC News:

• Judge: Prosecutor's rejection of gay juror 'shocking'
• George Zimmerman's old Myspace page includes slurs against Mexicans
• Chicago pays $45 million in 3 years to settle complaints against cops
• Maryland court finds pit bulls are 'inherently dangerous'
• NJ mom arrested after allegedly taking daughter, 5, tanning

Follow US News on msnbc.com on Twitter and Facebook