- NEW: Power is knocked out to one Port-au-Prince hospital, an official says
- NEW: Strong winds and rain are pounding the capital city, a hospital official says
- The eye of Tropical Storm Isaac is 55 miles southeast of Port-au-Prince, forecasters say
- The front of the storm is lashing the town of Jacmel on Haiti's southern peninsula
Port-au-Prince, Haiti (CNN) -- Tropical Storm Isaac lashed Haiti's southern peninsula early Saturday, bringing gale-force winds and potential flooding to a nation where hundreds of thousands still live in tent cities following a 2010 earthquake.
The eye of the storm was 55 miles southeast of Port-au-Prince, though its ferocity was felt hours ahead of its arrival as strong winds and high surf pounded Haiti's southern peninsula.
"It's going to be a brutal night," said CNN meteorologist Chad Myers.
By early Saturday, Isaac -- with maximum sustained winds of 70 mph -- was 55 miles southeast of Port-au-Prince, and moving at 13 mph, according to the Miami-based National Hurricane Center. A tropical storm is considered to have made landfall when its eye moves across the coast.
Rainfall accumulations of 8 to 12 inches are expected, with up to 20 inches possible in portions of Hispaniola -- the island shared by Haiti and the Dominican Republic.
The island is under a hurricane watch, though forecasters predict the earliest Isaac could hit hurricane strength would be Sunday when the storm tracks into the Gulf of Mexico.
Much of south Florida was under a tropical storm warning early Saturday, and a hurricane watch was issued for the Florida Keys, the National Hurricane Center said.
Isaac making Haiti landfall Forecasters believe Isaac also poses a risk to Tampa, Florida, where the Republican National Convention is due to kick off Monday.
Haiti: Racing to relocate people For the moment, though, those believed to be at greatest risk from Isaac's onslaught are some of the roughly 300,000 Haitians still living in temporary homes -- primarily tents -- following the January 2010 earthquake that killed more than 200,000. At the height of the disaster, it forced more than 500,000 people into tent cities.
Sean Penn talks about risk for Haiti Haitian President Michel Martelly said he is visiting camps to encourage people to go to shelters, but acknowledged not everyone would be able get out.
Tropical storm Isaac nears Haiti "Those who are very vulnerable, they are moved out of these camps. And the ones who are remaining behind are those who are stronger to fight this situation," he said.
Power is out at the Bernard Mevs Hospital in Port-au-Prince, which has been operating on a generator since just before midnight, said Scott Gillenwater of the Project Medishare, which provides services at the hospital.
Outside, strong winds and rain pounded the capital city.
Tampa RNC storm scenario "I can't imagine tents being able to last through this," he said.
Haiti in hurricane danger Staff at Bernard Meds spent three days preparing for the storm's arrival.
"We anticipate many crush injuries from mudslides to come into the trauma center," Gillenwater said.
Some residents living in one tent community said they were staying put with their belongings and would ride out the storm.
The front of the storm brought heavy rain and strong winds to Jacmel on Haiti's coast early Saturday.
Residents in Jacmel were worried Isaac would bring a repeat of the flooding the town experienced during a storm several years ago.
"I'm very worried about the water coming off the mountains and that the city fills up like a sink," said Hugues Paul, the mayor.
Large amounts of rainfall will cause mudslides and runoff that can block roads, or worse.
"We watch those storms every single time they come near because Haiti is so vulnerable," said Amy Parodi, a spokeswoman for the Christian humanitarian organization World Vision.
The agency has met with the government in the past to discuss contingency plans for major storms and ensure pre-positioned relief items are available, she said.
Elsewhere, storm preparations were under way in anticipation of Isaac's arrival.
Officials in Monroe County, in far south Florida, said three shelters would open Saturday for people who did not want to ride out the storm in their homes. They did not order a visitor evacuation as Isaac is forecast to cross the Florida Keys as a tropical storm, not a hurricane.
Isaac, which has already forced airline companies to cancel a couple dozen flights, should weaken as it moves through Haiti and Cuba.
Florida Gov. Rick Scott said it will be up to organizers of the Republican National Convention to decide the fate of the event.
While Isaac's path remains uncertain, the latest tracking information shows it crossing near the western Florida Keys and staying well west of Tampa, and not reaching hurricane strength until sometime Sunday or Monday.
A five-day projection shows Isaac making landfall near Pensacola, Florida, by early Wednesday.
Even though most of the state may catch a break, officials are taking the threat seriously.
"It has been a fortunate seven years since Wilma hit Florida," National Hurricane Center Director Rick Knabb said, referring to the last hurricane to make landfall in the state. "The luck is going to run out at some point."
CNN's Karen Smith, Gary Tuchman, Martin Savidge and Danielle Dellorto contributed to this report.
