Haitians are preparing for the arrival of Tropical Storm Isaac, as the storm is expected to make landfall on Friday night, the US National Hurricane Center has said.
On Friday afternoon, torrential rains began falling on the Dominican Republic, which shares the island of Hispaniola with Haiti.
Life-threatening flash-floods and mudslides, which are common in Haiti, could add to the misery of about 400,000 people still living in tent cities and camps after the January 2010 earthquake that killed more than 250,000 people.
The NHC said Isaac was centred about 160km south-southeast of the seaside Haitian capital Port-au-Prince late on Friday afternoon, packing top sustained winds of 100km per hour.
The storm is forecast to "move near or over southeastern Cuba on Saturday, move near or over central Cuba on Saturday night and approach the Florida keys on Sunday", the NHC said in its 21:00 GMT bulletin on Friday.
At that time, Isaac was gathering strength and speed some 160km southeast of Port-au-Prince, it said.
Long lines formed outside supermarkets in the Haitian capital as people filled stores to stock up on supplies.
Those still without proper shelter after the quake "remain amongst the most vulnerable, should the storm hit the city," said Jean-Claude Mukadi, Haiti's national director for the humanitarian group World Vision.
"Without a stable sanitation system or permanent housing, heavy rain and wind can create much larger problems like disease from water contamination."
President Michel Martelly, who cancelled a trip to Japan, earlier took to the airwaves with safety advice and to urge Haitians to follow the directions of civil defence personnel.
Laurent Lamothe, the Haitian prime minister, said the whole government, including security forces, has mobilised to prepare for the storm. "We are going to work with our international partners to co-ordinate response actions," Lamothe said.
The Haiti director for Oxfam, another humanitarian group, said that his group was preparing clean water and hygiene kits to help prevent the spread of cholera and other water-borne diseases.
A hurricane watch was in effect for Haiti, and a tropical storm warning was in effect for the neighbouring Dominican Republic and eastern Cuba.
A tropical storm watch was also in effect for the Florida Keys and parts of south Florida in the US. Forecasters put the entire coast of south Florida under tropical storm watch as of 5pm EDT (21:00 GMT) on Friday.
The storm could dump up to 51 centimetres of rain on Hispaniola.
Haiti has always struggled to cope with the aftermath of natural disasters. Deforestation has made the country highly vulnerable to landslides and flash flooding, and since the earthquake of 2010, the situation is even more serious.
"These rains could cause life-threatening flash floods and mudslides," the NHC said.
If this storm follows the forecast, then it will cause widespread flooding and potentially landslides as well.
Residents in the neighbouring Dominican Republic and on nearby Puerto Rico rushed to erect defences against the expected wind and rain, set to sweep on to Cuba and the southern US by the weekend.
Isaac has already churned over the tiny Caribbean islands of Guadeloupe, Martinique and Dominica.
Flights were cancelled, and some restaurants were closed, but fortunately Isaac wasn’t particularly powerful when it hit the islands. Generally the winds were under 75kph, and no more than 80mm of rain was reported.
As a precaution, hearings at the US military base at Guantanamo Bay in Cuba have been postponed.
It is thought Isaac may hit Florida on Monday - the same day the Republican National Convention starts.