Floodwaters have swamped streets in New Orleans' French Quarter and prompted a tornado warning in the area amid concerns that even worse weather is on the way to Louisiana and other states along the Gulf of Mexico.
The storms were associated with a broad area of disturbed weather in the Gulf that is expected to become better organised by this weekend when it threatens the region with torrential rain.
The weather system could then push the already swollen Mississippi River precariously close to the tops of levees that protect New Orleans, forecasters said.
The low pressure area was over water, south of the Florida Panhandle, on Wednesday. It was expected to strengthen into a storm as it moved west through the Gulf's warm waters.
Lines of thunderstorms associated with the system extended far out into the Gulf and battered New Orleans, where at least three inches of rain had fallen by mid-morning.
It is expected to get much worse in the days ahead.
Parts of Louisiana could see up to 12 inches of rain by Monday, with heavier amounts possible in some spots, forecasters said.
Mississippi and Texas were also at risk of torrential rain.
The National Weather Service said New Orleans is protected to a river level of 20 feet, but it was forecast to rise above flood stage to 19 feet by Friday.