SAN JUAN, Puerto Rico (AP) — Tropical Storm Melissa lumbered through the Caribbean Sea on Thursday, bringing a risk of dangerous landslides and life-threatening flooding to Jamaica and southern Hispaniola. Officials urged residents of flood-prone areas to seek higher ground.

Dozens of people were already in shelters in the Dominican Republic, and schools, businesses and government agencies were closed in the nine provinces under alert. Dozens of water supply systems were out of service Wednesday, affecting more than half a million customers.

In Jamaica, officials said 881 shelters would be made available as needed. Courts were ordered closed and schools were to switch to remote classes Thursday.

Evan Thompson, director of Jamaica’s Met Service, said the island’s eastern region could see up to 12 inches (30 centimeters) of rain. “Now that is significant rainfall, and that is the main thing that we should be mindful of at this time,” he said.

Similar rainfall amounts were expected for southern Haiti and the southern Dominican Republic through Saturday, with even more rain possible locally depending on Melissa's path later in the week. Heavy rain was also forecast forecast for western Jamaica, southern Hispaniola, Aruba and Puerto Rico.

People were also concerned about the impact in Haiti, which shares the island of Hispaniola with the Dominican Republic and has been devastated by past storms. Gang violence, poverty and ineffective governance mean storm preparations are limited.

Late Wednesday night, Melissa had maximum sustained winds of 50 mph (85 kph) and was moving west at 2 mph (4 kph), the U.S. National Hurricane Center in Miami said.

The slow-moving storm was centered about 335 miles (535 kilometers) south-southwest of Port-au-Prince, Haiti, and some 295 miles (475 kilometers) south-southeast of Kingston, Jamaica.

Melissa was expected to move closer to Jamaica and southwestern Haiti later this week. It was forecast to strengthen gradually and could be a hurricane by Friday and a major hurricane by the late weekend.

“Unfortunately, it is becoming increasingly likely that Melissa will become a large and dangerous hurricane,” the U.S. center warned.

Melissa is the 13th named storm of the Atlantic hurricane season, and the first named storm to form in the Caribbean this year.

The National Oceanic and Atmospheric Administration had predicted an above-normal season with 13 to 18 named storms. Of those, five to nine were forecast to become hurricanes, including two to five major hurricanes, which pack winds of 111 mph or greater.

The Atlantic hurricane season runs from June 1 to Nov. 30.