Puerto Rico is readying for a tropical storm after the National Hurricane Center on Wednesday issued a warning that also affects the U.S. Virgin Islands.

The storm, named Erika, which formed on the heels of Hurricane Danny on Monday, may bring sustained winds of up to 40 miles per hour to the American island territories, the Miami Herald reported. Erika formed in the middle of the Atlantic and was moving west on Wednesday morning, passing about 335 miles east of Antigua.

"A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area, generally within 36 hours," the Miami-Dade County, Florida-based National Hurricane Center explained. "Tropical storm conditions are expected to first reach the warning area ... tonight, and reach the Virgin Islands and Puerto Rico on Thursday."

Erika is likely to produce significant rainfall with accumulations of 2-4 inches, the center said. In some areas of Puerto Rico and the U.S. Virgin Islands, meanwhile, precipitation could reach levels of up to 8 inches.

"We will see this system as more of a rainfall event," the center's Ernesto Morales told El Nuevo Día. "The system will have winds, but the focus will be on the rain."

Meanwhile, forecasters are uncertain of how Erika might develop within the next few days because hurricane tracks have split on what path the tropical storm will take, the Miami Herald noted. Some predict it will move to the north, which would likely increase its strength, while others expect a more southern path, which would lead it to weaken as it moves over land or encounters drier conditions.

Whichever the outcome, Erika is unlikely to be "upgraded" before the end of the weekend, National Hurricane Center spokesman and meteorologist Dennis Feltgen said. Still, beyond the U.S. islands, residents in southern Florida might want to keep tabs on how the storm develops.

"We don't even have a hurricane until Sunday afternoon, and there's a great deal of uncertainty about that, so the best thing for South Florida residents to do is check back every day or so," Feltgen said.