(CNN) - Tropical Storm Isaac may not hit Haiti at hurricane strength, but that does little to assuage aid groups who fear another disaster in the vulnerable country.
Isaac is expected to whip the Caribbean nation with gale-force winds and stinging rain later on Friday.
"Our experience in Haiti clearly indicates that it is not the storm or the winds, it's the rain that causes the problems," said Sinan Al-Najjar, the Red Cross' deputy country representative in Haiti. "When rain comes, landslides and flash floods do happen in Haiti. We are trying to focus on which are the flood areas, which are the risk areas."
Hundreds of thousands of people left homeless by the devastating 2010 earthquake continue to live in camps.
With floodwater comes the risk of another outbreak of cholera, an infection of the large intestine that causes severe diarrhea.
"After floods, it's going to be almost certain that we see increases in cholera cases," Al-Najjar said. "We already witnessed that with the few weeks of rain we had in April. We had spikes due to daily rain. If a flood comes, we know certainly cholera is going to be an issue."
A revised National Hurricane Center forecast map shows the eye of the storm slipping just south of the Dominican Republic on Friday morning before making landfall in western Haiti later in the day. The neighboring nations share the island of Hispaniola.
As of 8 a.m. ET, Isaac was 220 miles southeast of Port-au-Prince, Haiti. It had maximum sustained winds of 50 mph, the hurricane center in Miami said.
The storm was moving west at 15 mph.
Rainfall accumulations of 8 to 12 inches are expected, with maximum amounts of 20 inches possible over Hispaniola.
"These rains could cause life-threatening flash floods and mud slides," forecasters warned.
Many of the Haitians living in camps had no idea that a storm was coming, CNN correspondent Gary Tuchman reported from Port-au-Prince. Not until a translator told them that Isaac was nearing did people in the streets know of the storm'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 Port-au-Prince.
"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.
The forecast map shows Isaac crossing the nation as a tropical storm with winds under 74 mph, probably in the 50 mph range.
"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 previous summers to discuss contingency plans for major storms, and pre-positioned relief items are available, she said.
The storm is expected to cross the eastern half of Cuba on Saturday, also as a tropical storm.
Isaac also poses a risk to the Republican National Convention in Tampa, Florida. Gov. Rick Scott said Thursday it will be up to organizers 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 Monday. A five-day projection shows Isaac making landfall near Pensacola, Florida, by early Wednesday.
Even the though most of the state may catch a break, officials are taking the potential threat seriously.
"Obviously, we hope Isaac doesn't hit Florida, but we must take every precaution," Scott said.
While the convention will have the final say on any changes to the planned event, organizers are working with state and local officials to ensure everyone has the same information, the governor said.
But Tampa is not the only part of Florida that could be hit, and Scott urged Floridians to be prepared.
"What everyone needs to do is to starting tracking it, watch the weather, heed any warnings, get ready, get 72 hours' worth of supplies," he said.
Tampa Mayor Bob Buckhorn said his city is well prepared for the 50,000 people headed there for the Republican National Convention, which starts Monday.
The Florida National Guard is ready to back up authorities around the state.
CNN's Gary Tuchman, Nelson Quiones, Dave Alsup, Allison Brennan, Kevin Liptak, Jason Hanna, Brad Lendon, Dave Hennen, Sean Morris and Mariano Castillo contributed to this report.