The US National Hurricane Center has issued a warning for the northern Gulf of Mexico coast.
Also a strong magnitude-7.4 earthquake has struck off the coast of El Salvador and a tsunami warning has been issued.
Isaac is still a tropical storm moving past the Florida Keys, but it is heading into the warm waters of the Gulf of Mexico where it could gain strength.
Many areas have declared a state of emergency including Louisiana, Mississippi and Alabama.
Meanwhile, there were no immediate reports of damages or injuries after the quake struck today.
The earthquake was located 86 miles south-south west of San Miguel, the US Geological Survey reported on its website and at a depth of 32.9 miles.
A tsunami warning is in effect for Costa Rica, Nicaragua, El Salvador, Honduras, Guatemala, Panama and Mexico.