(CNN) -- Tropical Storm Rafael churned in the central Atlantic early Monday as it neared hurricane strength, forecasters said.
Maximum sustained winds increased to 70 mph late Sunday, according to the National Hurricane Center in Miami. The storm was centered about 290 miles (465 km) north-northeast of San Juan, Puerto Rico, and was swirling north-northwest at 10 mph.
"Some strengthening is forecast ... and Rafael is expected to become a hurricane later today," the hurricane center said.
A projection map shows Rafael headed toward Bermuda later this week.
Officials there issued a tropical storm watch Sunday and told residents that they expected the storm to affect the British territory on Tuesday. A tropical storm watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.
While the National Hurricane Center predicted Rafael will stay well to the east of the Bahamas through Monday night, it did warn of life-threatening surf conditions and rip currents on the eastward-facing beaches of the Bahamas over the next few days.
Forecasters expect Rafael to leave between 4 to 8 inches of rain over the northern Leeward Islands, with some pockets getting as much as 12 inches.
"These rains could cause life-threatening flash floods and mudslides, especially in areas of mountainous terrain," the hurricane center said.
Rainfall amounts of 1 to 3 inches are possible in the Virgin Islands, Culebra and Vieques, forecasters said.
Read more: Tropical Storm Leslie expected to pass east of Bermuda
CNN's Chandler Friedman contributed to this report.