[Read Thursday’s live updates on Hurricane Dorian.]

Tropical Storm Dorian was gaining strength in the Atlantic Ocean and moving toward Barbados and the eastern Caribbean on Monday. The storm could develop into a hurricane as it closes in on Puerto Rico and the Dominican Republic this week, forecasters said.

The National Hurricane Center said Monday afternoon that Dorian, the fourth tropical storm of the Atlantic hurricane season, was about 60 miles east-southeast of Barbados. It was moving west at 14 miles an hour, with maximum sustained winds measured at 60 miles an hour.

A hurricane watch was in effect for St. Lucia. And a tropical storm warning was in effect for Barbados, St. Vincent and the Grenadines, where Dorian is expected to dump 3 to 8 inches of rain.

By Wednesday, forecasters said, Dorian could sideswipe southwestern Puerto Rico as a Category 1 hurricane, with winds of at least 74 miles an hour, and then make landfall in the Dominican Republic early Thursday.