Tropical Storm Melissa moved slowly through the Caribbean Sea Thursday, raising concerns of dangerous landslides and life-threatening flooding in Jamaica and southern Hispaniola. Officials urged residents in flood-prone areas to seek higher ground.

In the Dominican Republic, dozens of people were already in shelters. Schools, businesses, and government offices were closed in nine provinces under alert. Over half a million customers were affected by outages in dozens of water supply systems.

Jamaican authorities announced that 881 shelters would be available as needed. Courts were closed, and schools shifted to remote learning Thursday. Evan Thompson, director of Jamaica’s Meteorological Service, warned that the island’s eastern region could see up to 12 inches (30 centimeters) of rain.

Similar rainfall was expected for southern Haiti and the southern Dominican Republic through Saturday, with potential for even higher local amounts depending on Melissa’s path. Heavy rain was also forecast for western Jamaica, southern Hispaniola, Aruba, and Puerto Rico.

Concerns were particularly high for Haiti, where gang violence, poverty, and ineffective governance limit storm preparedness.

As of early Thursday, Melissa had maximum sustained winds of 50 mph (80 kph) and was moving west-northwest at 3 mph (5 kph), according to the U.S. National Hurricane Center. The storm was centered about 300 miles (485 kilometers) south-southwest of Port-au-Prince, Haiti, and 240 miles (390 kilometers) south-southeast of Kingston, Jamaica.

Melissa is expected to approach Jamaica and southwestern Haiti later this week, gradually strengthening and potentially reaching hurricane status by Friday, and major hurricane strength by the weekend. The U.S. center cautioned that Melissa could become a “large and dangerous hurricane.”

Melissa is the 13th named storm of the Atlantic hurricane season and the first to form in the Caribbean this year. The National Oceanic and Atmospheric Administration predicted an above-normal season, forecasting 13 to 18 named storms, five to nine hurricanes, and two to five major hurricanes.

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