Tropical Storm Sandy formed Monday afternoon in the southwestern Caribbean Sea about 300 miles south of Jamaica.
The National Hurricane Center said at 11 p.m. Monday that the storm had maximum sustained winds of 45 miles per hour and was nearly stationary.
The most reliable computer models predict the storm will eventually track north, then northeast, possibly affecting Jamaica, Cuba, Hispaniola and the Bahamas.
Conditions are more favorable for development of tropical storms in the Caribbean and Gulf of Mexico this time of the season.
With about a month left in the 2012 hurricane season, forecasters also are watching a second system in the south central Atlantic Ocean that appears likely to develop.
The hurricane center gives that system a 50 percent chance of developing into a tropical storm within the next 48 hours.
It is heading northeast about 10 mph and probably would not threaten land.