Mayor of New Orleans LaToya Cantrell has declared a state of emergency. Mandatory evacuation orders are being enacted elsewhere in Louisiana.
Tropical storm Barry is expected to make landfall late on Friday into Saturday morning (12-13 July), hitting coastal areas hard.
Thunderstorms and flash floods have already struck New Orleans, disrupting transport links - including flights.
"This is going to be a major weather event for a huge portion of the state," Louisiana governor John Bel Edwards has warned.
Storm surge warnings have been issued in south-east Louisiana, ABC News reports, and a storm surge watch elsewhere along the Louisiana and Mississippi coast.
The FCO issued new travel advice for the United States earlier this week, warning: “Severe thunderstorms have caused flooding in New Orleans.
“A tropical storm is also forecast in the Gulf of Mexico, which could affect the coastal areas of Louisiana and Alabama from Friday 12 July.
“You should monitor local weather updates from the National Weather Service and follow the advice of local authorities.”