The Mississippi Emergency Management Agency is telling people in or near the three coastal counties to prepare for tropical weather by late Friday or early Saturday.
The National Hurricane Center in Miami says Tropical Storm Karen has formed in the Gulf of Mexico. It was about 485 miles south of the mouth of the Mississippi River and already had maximum sustained winds of 65 mph.
A hurricane watch was put in effect Thursday from Grand Isle, La., to Indian Pass in the Florida Panhandle, an area that includes Mississippi's and Alabama's entire coastline.
Karen could be at or near hurricane strength by Friday.
MEMA spokesman Greg Flynn says people who live in flood-prone areas should think about where they'll go if ordered to evacuate. He says south Mississippi residents, particularly along the coast, need to have family communication plans and make sure they have supplies in case power goes out.