A storm has swamped New Orleans streets and paralysed traffic as concerns grew that even worse weather was on the way in the form of a possible hurricane that could strike the Gulf Coast and raise the Mississippi River to the brim of the city's protective levees.
The storm was associated with a broad area of disturbed weather in the Gulf that forecasters said was on track to strengthen into a hurricane by the weekend.
The system was expected to become a tropical depression by Thursday morning, a tropical storm by Thursday night and a hurricane on Friday, according to the National Hurricane Centre.
Lines of thunderstorms associated with the system ranged far out into the Gulf and battered New Orleans, where as much as 18cm of rain fell over a three-hour period on Wednesday morning, forecasters said.
Mississippi and Texas were also at risk of torrential rains.
Mississippi Governor John Bel Edwards declared a state of emergency and said National Guard troops and high-water vehicles would be positioned all over the state.
'The entire coast of Louisiana is at play in this storm,' Edwards said.
Forecasters said Louisiana could see up to 30cm of rain by Monday, with isolated areas receiving as much as 46cm.
The heavy additional rain could push the already swollen Mississippi River precariously close to the tops of levees that protect New Orleans, officials said.
© AAP 2019