Bertha was 75 miles east of the Mississippi River mouth early Wednesday and was forecast to make landfall in southeastern Louisiana by Wednesday night.
50-mph sustained winds were the main threat, with storm surge up to 4 feet in parts of Louisiana and 2 to 4 inches of rain from western Florida to southern Louisiana.
Louisiana, Mississippi and Florida opened sandbag sites, while double red flags closed Gulf waters to swimmers and tropical storm warnings stretched from the Florida Panhandle to Morgan City, including New Orleans.
Mississippi communities still recovering from Tropical Storm Arthur flooding were watching Bertha closely, though no shelters had opened and no evacuations had been ordered by Tuesday.
Bertha is the Atlantic season's second named storm and was expected to weaken after moving inland toward Texas.