Isaac has now been downgraded from a hurricane to a tropical storm.
Forecasters expect it to weaken further over the next two days.
It has already wreaked havoc across the US state of Louisiana.
Heavy rains and high winds have triggered huge floods and knocked out power for half-a-million homes and businesses.
A curfew remains in force in New Orleans, which has been lashed by heavy rains since Wednesday morning.
Multi-billion-dollar flood defences erected after the Katrina disaster seven years ago were not breached, although seawater did flow over one levee further south of the city.
The White House has declared an emergency in Louisiana and nearby Mississippi, allowing federal funds to be released to both states.