According to the BBC reports, Isaac is heading for New Orleans, possibly as early as Tuesday night, nearly seven years to the day after Hurricane Katrina devastated the city.
Isaac killed at least 24 people in Haiti and the Dominican Republic.
The storm also caused flooding and damage in the Caribbean.
Late on Monday, the National Hurricane Centre (NHC) warned Isaac could reach category two strength, with top winds of 100mph (160km/h). The forecast was revised up from category one.
President Obama approved Louisiana’s request for a federal disaster declaration, making available federal funds for recovery activities such as clearing debris.
The governors of Louisiana, Florida, Mississippi and Alabama declared emergencies in their states.
The Republican governors of Alabama, Louisiana and Mississippi have cancelled their trips to their party’s convention to focus on disaster prevention efforts.
The weather officials warned that Isaac is already a large storm and could bring significant damage to areas within hundreds of miles of its center.
The NHC said that at 20:00 EDT (20:00 GMT) on Monday, Isaac was centered about 230 miles (370km) south-east of the mouth of the Mississippi river, with maximum sustained wind speeds of 70mph (110km/h).
The storm is moving forward at about 10mph and storm winds extend out about 205 miles from the center.
The NHC warned that wind speeds could reach between 96-110mph before the storm makes landfall.
Evacuations have already been ordered for some low-lying Louisiana parishes and parts of coastal Alabama.
Wednesday will be the seventh anniversary of Hurricane Katrina, which strengthened in the Gulf to a category five storm, before weakening to category three by the time it reached New Orleans.
Federal officials said the levees around New Orleans are now equipped to handle storms stronger than Isaac.
According to the Federal Emergency Management Agency Administrator Craig Fugate, “It’s a much more robust system than what it was when Katrina came ashore,”
Fugate also said that Isaac was not just a New Orleans storm.
“This is a Gulf Coast storm. Some of the heaviest impact may be in Alabama and Mississippi,” he said.
Vehicles were left at New Orleans on the highway heading west for Baton Rouge on Monday, as people made their way to higher ground.
A hurricane warning is already in effect for some 300 miles of the Gulf Coast in four states from Louisiana to Florida, with lower-level warnings issued for many areas along Florida’s west coast.
Governor Rick Scott of Florida said 60,000 people were already without power as a result of the storm.