SAN JUAN, Puerto Rico (AP) — Forecasters on Tuesday issued a tropical storm warning for Puerto Rico and other parts of the Caribbean, saying a disturbance in the Atlantic is likely to soon strengthen into a tropical storm that might reach the U.S. mainland.

The storm, which could be named Isaias if it gains force, was expected to move through the Leeward Islands on Wednesday, and near or over the Virgin Islands and Puerto Rico on Wednesday night. It would be over Hispaniola on Thursday.

The U.S. National Hurricane Center in Miami issued warnings for Puerto Rico, the British and U.S. Virgin Islands, Antigua, Barbuda, Dominica, Martinique , Montserrat, St. Kitts and Nevis, Guadeloupe, Martinique, St. Martin, St. Maarten, Saba, St. Eustatius and portions of the Dominican Republic. A tropical storm warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

The long-term forecast showed the storm headed toward the southeastern U.S., though the Hurricane Center said, “It cannot be stressed enough that since the system is still in the formative stage, greater than average uncertainty exists regarding both the short-term and longer-term track and intensity forecasts.”

Officials in Puerto Rico said in a news conference that they were concerned about landslides and widespread flooding and urged people not to lower their guard, noting that the U.S. territory is struggling with a spike in COVID-19 cases. The island also is still trying to recover from 2017's devastating Hurricane Maria and a string of earthquakes earlier this year that damaged or destroyed hundreds of homes in the island's south.

“We're not facing a situation like Maria, but we have to remain wary,” said Pedro Janer, secretary of the Department of Public Safety.

Many worried about whether Puerto Rico's fragile power grid, which was destroyed by Maria, would withstand tropical storm force winds. On Tuesday, the island's power company and union leaders announced that more than 450,000 customers were briefly left without electricity when a plant was knocked offline for unknown reasons.

During a press conference late Tuesday, Gov. Wanda Vázquez said the storm is expected to cause power outages. She added that more than 300 shelters across the island are prepared to receive people if needed and that more than 130,000 face masks were available.

“We've lived through several emergencies at one time,” she said. “I want you to remain calm.”

On Tuesday night, the storm system was centered 340 miles (545 kilometers) southeast of the Leeward Islands. It was moving west-northwest at 25 mph (40 kph).

The maximum sustained winds were 40 mph (65 kph) with higher gusts. Forecasters said some strengthening was expected during the next 48 hours, with the system expected to become a tropical storm Tuesday night or Wednesday.

Forecasters estimated the northern Leeward Islands, British and U.S. Virgin Islands and Puerto Rico would experience 3 to 6 inches (8 to 15 cm) of rainfall with maximum amounts of 10 inches (25 cm).