Isaac is producing a dangerous storm surge along the northern Gulf Coast, and flooding from rainfall is to follow, the US National Hurricane Center said.
The NHC replaced the Tropical Storm watch from west of Cameron, Louisiana to Sabine Pass, Texas with a tropical storm warning in its latest advisory.
"Little change in strength is forecast tonight. Slow weakening is expected after that," it said.
Hurricane Isaac crashed ashore in southeast Louisiana on Tuesday, bringing high winds and soaking rains that pose the first test for multi-billion-dollar flood defenses put in place in New Orleans after Hurricane Katrina devastated the US Gulf Coast seven years ago.