US PRESIDENT Barack Obama has warned of significant damage and flooding from Tropical Storm Isaac, telling people in its path to take the “big storm” seriously and follow directions.

“I want to encourage all residents of the Gulf Coast to listen to your local officials and follow their directions, including if they tell you to evacuate,” Mr Obama said in a televised statement at the White House.

“We’re dealing with a big storm and there could be significant flooding and other damage across a large area. Now is not the time to tempt fate. Now is not the time to dismiss official warnings. You need to take this seriously.”

Mr Obama noted that the storm, forecast to become a hurricane, was expected to make landfall later in the day, and said he had managed a wide-ranging effort by federal and local governments to make preparations.

His appearance was a reminder of the power of an incumbent president to intervene at politically advantageous moments, as he projected an image of strength and competence, just as Republicans met for their National Convention.

The jamboree to nominate White House candidate Mitt Romney in Tampa, Florida, will try to create the opposite impression of Mr Obama, making the case that he has failed in the Oval Office.

Isaac is bearing down on New Orleans seven years after the Bush administration’s botched handling of the much bigger Hurricane Katrina became a political disaster and left a doom-laden case study for future presidents.

Mr Obama said he had ordered the Federal Emergency Management Agency to be in place more than a week ago to coordinate with local officials from Puerto Rico to Florida, Louisiana and other Gulf of Mexico states.

“Right now, we already have response teams and supplies ready to help communities in the path of the storm,” he said.

“I’ll continue to make sure that the federal government is doing everything possible to help the American people prepare for and recover from this dangerous storm.

“As we get additional updates from the Hurricane Centre, as well as from FEMA in terms of activities on the ground, we’ll be providing continuous updates both local and national levels.”

The US National Hurricane Centre said in its 1200 GMT (10pm AEST) advisory that Isaac was on the verge of becoming a hurricane, warning of a “significant storm surge and freshwater flood threats to the northern Gulf Coast”.

The Miami-based NHC predicted the centre of the massive storm would be over the Louisiana coast by late Tuesday or early Wednesday, saying “preparations to protect life and property should be rushed to completion”.