The U.S. National Hurricane Center in Miami said Tomas strengthened Saturday night with 90 mph (150 kph) winds and was centered about 50 miles (85 kilometers) west-southwest of St. Lucia. It was moving west-northwest at 9 mph (15 kph).
Tomas, the Atlantic season’s 12th hurricane, was expected to drop up to 6 inches (15 centimeters) of rain in the region.
Forecasters said it could become a Category 2 storm Monday evening and possibly reach Category 3 by midweek, with winds around 115 mph (184 kph).
It was forecast to head toward Jamaica and could unleash heavy rains in southern portions of the Dominican Republic and Haiti, which is struggling to recover from a devastating earthquake and cope with a recent cholera outbreak.
Haiti issued an orange storm alert, the second highest level. Authorities warned southern and western regions — including the quake-ravaged capital of Port-au-Prince, where an estimated 1.3 million people are living in tent camps — to be on guard for high winds, thunderstorms and possible flooding.