The storm was expected to brush Jamaica's eastern provinces and then regain hurricane strength before passing near the western coast of Haiti early Friday morning with heavy rains, according to the U.S. National Hurricane Center in Miami.
Haiti's government urged evacuation of the emergency camps set up after the Jan. 12 earthquake.
As the skies darkened over Port-au-Prince and roof-tarps started flapping in the wind Thursday morning, a policeman at the Corail-Cesselesse camp shouted through a megaphone: "The hurricane is not a joke! ... You need to get out of here!"
Survivors of the devastating earthquake have fought forced evictions, weathered storms, organized themselves into security committees, and rallied for better services and aid. Now they are being told to leave - and few have anywhere to go.
The government says more than 1,000 shelters are available, but that can refer to any building expected to stand up to high winds. The U.N. Office for the Coordination of Humanitarian Affairs said there is a need to identify safe potential storm shelters.
Painfully slow reconstruction from the quake, prior storms and the recent commitment of government resources to fight a growing cholera epidemic have left people with few options as overtaxed aid workers struggle to help.
"We are using radio stations to announce to people that if they don't have a place to go, but they have friends and families, they should move into a place that is secure," said civil protection official Nadia Lochard, who oversees the department that includes the capital, Port-au-Prince.
Fear and confusion have swept through many of the camps. Tensions boiled over into scuffles Wednesday at the Corail camp when managers tried to explain a planned voluntary evacuation of nearly 8,000 people from ShelterBox tents once promised to be hurricane-resistant.
The tentative plan there, as at several other camps, is to move some people to schools, churches, and other structures such as abandoned prisons. But most of the homeless are being told to seek out friends or family who can take them in.
As news of Tomas' predicted arrival slowly filtered through Port-au-Prince via windup radios and megaphone announcements, unease set in among people who already lost homes and loved ones in the quake and saw their tents ripped apart in lesser storms this year.
"The tension is elevated. People are really concerned about their belongings. They're posing a lot of legitimate questions," said Bryant Castro, a American Refugee Committee staffer at Corail-Cesselesse.
Concerns are even greater in the western reaches of Haiti's southern peninsula, where heavy flooding is predicted.
Disaster officials have extended a red alert, their highest storm warning, to all regions of the country, as the storm is expected to wind its way up the west coast of the island of Hispaniola, which Haiti shares with the Dominican Republic, through storm-vulnerable Gonaives and Haiti's second-largest city, Cap-Haitien, sometime Friday.
By midday Thursday, Tomas had maximum sustained winds of 45 mph (kph). It was centered 295 miles (475 kilometers) west-southwest of Port-au-Prince and moving north at 8 mph (13 kph).
Tomas killed at least 14 people when it hit the eastern Caribbean island of St. Lucia as a hurricane on Saturday.
A hurricane warning was in effect for Haiti, the southeastern Bahamas, the Turks and Caicos Islands and the Cuban province of Guantanamo. A tropical storm warning was issued for Jamaica.
In Kingston, Jamaica, the Office of Disaster Preparedness said people in eastern provinces should evacuate low-lying areas. Schools were closed in Kingston, the capital, and surrounding parishes.
Jamaica is still struggling to recover from floods unleashed by Tropical Storm Nicole in late September that killed at least 13 people and caused an estimated $125 million in damage.
People who are still using boats to move about in the island's rural western regions also will be moved to shelters, said Ronald Jackson, of the emergency management office.