SAN JUAN, Puerto Rico — Puerto Rico activated the National Guard and canceled the start of classes in public schools as forecasters warned that the U.S. territory would be hit by Tropical Storm Ernesto, which formed in the Atlantic Ocean on Monday.

Tropical storm warnings were in effect for Puerto Rico, Vieques, Culebra, the U.S. and British Virgin Islands, Antigua, Barbuda, Anguilla, St. Kitts, Nevis, Montserrat, Guadeloupe, St. Martin, St. Barts and St. Maarten.

The storm was located about 230 miles east-southeast of Antigua late Monday. It had maximum sustained winds of 40 mph and was moving west at 28 mph. Ernesto is the fifth named storm of the Atlantic hurricane season.

This GOES-16 GeoColor satellite image taken at 4:30 p.m. EDT shows Tropical Storm Ernesto over the Atlantic Ocean and moving west-northwest toward the Lesser Antilles on Monday. National Oceanic and Atmospheric Administration via AP

Officials in the French Caribbean said the storm was expected to drench Guadeloupe on Monday and pass near St. Barts and St. Martin. The National Hurricane Center said Ernesto is forecast to move over or near Puerto Rico and the U.S. and British Virgin Islands on Tuesday evening.

Forecasters warned the storm is expected to unleash floods and landslides.

“We cannot let our guard down,” Nino Correa, Puerto Rico’s emergency management commissioner, said at a news conference.

Puerto Rico Gov. Pedro Pierluisi said more than 340 shelters across the island would be available if necessary and that more than 200 personnel with the National Guard were activated.

Ernesto Morales, with the National Weather Service in San Juan, said between six to eight inches of rain are expected, with higher amounts in isolated areas. He also warned of hurricane-strength wind gusts as the storm is expected to hit northeast Puerto Rico and move across the U.S. territory late Tuesday and early Wednesday.

He urged people to prepare and stay alert given ongoing uncertainties over the approaching system.

“This trajectory is not written in stone and will be changing,” he said.

Officials also warned Ernesto would cause widespread power outages given the fragile state of Puerto Rico’s power grid, which crews are still rebuilding after Hurricane Maria struck the island in September 2017 as a Category 4 storm.

“That’s a reality,” said Juan Saca, president of Luma Energy, a private company that operates the transmission and distribution of power in Puerto Rico.

Power outages also were a concern in the neighboring U.S. Virgin Islands for similar reasons. Even before the storm approached, officials announced island-wide blackouts on St. John and St. Thomas.

U.S. Virgin Islands Gov. Albert Bryan Jr. urged people to take the storm seriously.

“This is a practice run to make sure we’re really prepared,” he said, noting that the peak of hurricane season is yet to come.

Ernesto is expected to become a hurricane early Thursday as it turns north toward Bermuda, with some forecasters warning it could strengthen into a major Category 3 storm.

The National Oceanic and Atmospheric Administration has predicted an above-average Atlantic hurricane season this year because of record-warm ocean temperatures. It forecasted 17 to 25 named storms, with four to seven major hurricanes of Category 3 or higher.