The system was moving westward at about 15 miles per hour across the Central Atlantic. If it strengthens into a tropical storm, with winds of 39 to 73 mph, the NHC will name it Cristobal.
Weather models forecast the system will cross the Lesser Antilles and reach the central or western Caribbean Sea south of the Greater Antilles over the next five days or so.
The Lesser Antilles include the Caribbean Islands from the Virgin Islands south to Trinidad.
The Greater Antilles include Cuba, Jamaica, Hispaniola and Puerto Rico.
Energy traders watch for storms that could enter the Gulf of Mexico and threaten US oil and gas production facilities.
Commodities traders also watch storms that could hit agriculture crops like citrus and cotton in Florida and other states along the Gulf Coast.
(Reporting by Scott DiSavino; Editing by John Picinich)