MIAMI (AP) — Tropical Storm Rafael lashed the Leeward Islands with heavy rains Sunday, threatening to strengthen into a hurricane within a day, U.S.-based forecasters said.

The storm was centered at 8 a.m. EDT Sunday about 100 miles north-northwest of St. Martin and was gradually moving away from the Virgin Islands, the U.S. National Hurricane Center in Miami said. Rafael had top sustained winds of 50 mph and was moving north-northwest at 12 mph.

Forecasters said Rafael could become a hurricane by Monday, adding interests in Bermuda should monitor Rafael’s progress. The center’s advisory said a tropical storm or hurricane watch may be required for Bermuda sometime Sunday.

A tropical storm warning has been discontinued for Anguilla. Earlier Sunday, a tropical storm watch was discontinued for Puerto Rico.

Rafael could dump rainfall totaling between 3 and 5 inches on some islands and possibly up to 10 inches in isolated spots over the lesser Antilles and the Virgin Islands, raising the threat of flooding, forecasters said. Potentially heavy rains could cause life-threatening flash floods and mudslides, particularly in mountainous terrain, the hurricane center said.

Forecasters also warned of possibly dangerous surf churned up by Rafael.

Tropical storm-force winds extended outward up to 175 miles from the center of Rafael, mainly to the northeast and east of the system, forecasters said.