Fishermen near Jamaica's capital tied their wooden skiffs down along a rocky shore on Sunday as a poorly organized Tropical storm Ernesto spun off the Caribbean country's southern coast on a path that may carry it across the Yucatan Peninsula and into the Gulf of Mexico.

Emergency officials said some eastern parts of the island were already being drenched by rain from Ernesto's outer bands and there was expected to be heavy rainfall and gusty winds over the island. Tropical storm conditions were expected by the afternoon from the rapidly moving storm, though U.S. forecasters said it was becoming less organized. 

Jamaica's emergency management agency urged people in flood-prone areas to be on alert and avoid flooded waterways and submerged roads. The government had earlier ordered fishermen on outlying cays to evacuate and move to the main island.

Prime Minister Portia Simpson Miller called on all Jamaicans to make the necessary preparations to ensure their safety.

"I urge you to especially consider the children, the sick, persons with disabilities, the elderly, and the most vulnerable in the society," said Simpson Miller.

Daniel Edwards, a dreadlocked fisherman in Port Royal, a small fishing village at the tip of a spit of land near Kingston's airport, said vigorous lightning lit up the sky over the sea late Saturday and early Sunday.

Bailing out his small wooden fishing boat next to a dilapidated wooden dock, Edwards said he wasn't overly concerned about the tropical storm's passage.

"It's not much of a muchness," said the veteran fisherman, decked out in rain gear.

Ernesto was threatening to dump 76 to 152 millimetres of rain on the Caribbean island of less than 3 million people before drenching the coasts of Honduras, Belize and Mexico, possibly as a hurricane.

Forecasters said it might weaken into a storm over land and then re-emerge as a hurricane in the Gulf of Mexico late next week.

A tropical storm watch was in effect for the coast of Honduras, from the border with Nicaragua westward to Punta Castilla, and the island of Grand Cayman in the Cayman Islands. Showers and thunderstorms were also still possible over the Dominican Republic and Haiti, which share the Caribbean island of Hispaniola.

The U.S. National Hurricane Center in Miami said that Ernesto was centered about 335 kilometres south of Kingston, Jamaica, Sunday afternoon. It had maximum sustained winds near 85 kph and was moving swiftly westward at 37 kph.

The storm wasn't expected to strengthen much during the day. But it was forecast to gradually begin gaining power over the next two days and possibly reach hurricane strength by Wednesday.

Many Jamaicans stocked up on bottled water, batteries and canned goods in the southern capital of Kingston.