Miami, Florida - Tropical Storm Ernesto advanced Tuesday through the Caribbean toward Puerto Rico, where officials shut down schools and sent workers home as forecasters warned of a possible hurricane.

The storm was moving west-northwest toward the US island with maximum sustained winds of 60 miles per hour, according to the National Hurricane Center.

It was expected to pass near or over the US and British Virgin Islands in the evening before moving to the north and northeast of Puerto Rico later Tuesday and Wednesday.

The storm could become a hurricane within 12 hours, the NHC said – likely after it moves north of Puerto Rico.

As a result, a hurricane watch had been issued for the Virgin Islands as well as Culebra and Vieques, two outlying islands off Puerto Rico.

Officials in Puerto Rico have already mobilized the National Guard, suspended classes in public schools, and sent home non-essential workers.

The storm could strengthen into a major hurricane over the next few days, the NHC said.

Scientists say climate change likely plays a role in the rapid intensification of storms because there is more energy in a warmer ocean for them to feed on.

Warmer-than-normal Atlantic waters threaten to supercharge this year's hurricane season, scientists have warned.