The possibility of stinging rain and tree-bending wind descending on the hundreds of thousands of Haitians living in camps has aid groups in the country on high alert.
As much of 12 inches of rain is forecast for some parts of the country, posing danger to the more than 400,000 Haitians in the camps.
Many of those people had no idea that a storm was coming, CNN correspondent Gary Tuchman reported from Port-au-Prince. Not until a translator told them that a potential hurricane was nearing did people in the streets know of Isaac's approach or that the government had opened some shelters. Residents of one tent community said they were staying put with their belongings and would ride out the storm.
There were no signs of hurricane preparations in the city, no buildings being boarded up in Port-au-Prince.
"These rains could cause life-threatening flash floods and mudslides," the National Hurricane Center in Miami said.
Isaac also poses a risk to the Republican National Convention in Tampa, Florida. The storm could hit anywhere in the state, Gov. Rick Scott said Thursday, and it will be up to convention organizers to decide the fate of the event.
But before it nears Florida, Isaac is forecast to run over Hispaniola, the island that's home to Haiti and the Dominican Republic.
"The risks are obvious in terms of flooding of low-lying areas," said Jean-Michel Vigreux, director for CARE International in Haiti.
Even though two-and-a-half years have passed since the earthquake, its impact on the country cannot be understated, he said.
Large amounts of rainfall will cause mudslides and runoff that can block roads, or worse.
"The country is still recovering from the earthquake. It's difficult to imagine that (a storm) will hurt the most vulnerable places again," Vigreux said.
As of 5 p.m. ET, Isaac was about 180 miles (290 kilometers) southwest of San Juan, Puerto Rico, the center said.
It was moving to the west-northwest near 16 mph with maximum sustained winds near 40 mph.
The center of the storm is forecast to approach the Dominican Republic on Thursday night and move over the south coast of Hispaniola on Friday, the center said.
While the storm is yet to reach hurricane strength, it has already delivered shock waves to the Caribbean, postponing a hearing for September 11 terrorist attack suspects at the U.S. naval base in Guantanamo Bay, Cuba.
"Strengthening is forecast during the next 48 hours, and Isaac could still become a hurricane on Friday before it reaches Hispaniola," according to forecasters.
Aid organizations were keeping an eye on Haiti. According to the U.N. High Commissioner for Refugees, 421,000 people are in camps in and around Port-au-Prince, the capital.