Storm Barry has been gathering speed over the Gulf of Mexico in recent days.
Officials say sustained wind speeds have grown to 50mph (80km/h) and may still reach hurricane strength by the time it makes landfall.
It is expected to bring a storm surge and heavy rainfall to the city of New Orleans - which has already seen thunderstorms and flash floods.
The National Weather Service warns that flooding from the slow-moving storm poses the greatest risk.
Between 10-20in (25-50cm) rain is forecast to hit the state, where the Mississippi River is already nearing flood levels.
The president's declaration frees up wide-ranging federal resources which can be used to help in the emergency situation.