Isaac achieved hurricane status shortly after US President Barack Obama warned of the likelihood of significant damage from flooding.
The storm reached hurricane strength on Tuesday with maximum wind speeds approaching 130km as it bore down on the northern US Gulf Coast, the National Hurricane Center said.
“Reports from an air force reserve hurricane hunter aircraft indicate that maximum winds associated with Isaac have increased,” the center said. “On this basis, Isaac is being upgraded to a hurricane.”
“The center of Hurricane Isaac should reach the coastline of southeastern Louisiana this evening,” the NHC later said, adding that Isaac had become better organised by Tuesday afternoon.
The US president told people in its path to take the “big storm” seriously and to follow directions, in a televised statement at the White House.
“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,” Obama said.
“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.”
Obama emphasised that the 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 convention.
The event to nominate White House candidate Mitt Romney in Tampa, Florida, will try to create the opposite impression of 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. It was moving northwestward towards the mouth of the Mississippi River.
“Isaac has finally formed into a hurricane, so we are officially in the fight and the city of New Orleans is on the
front lines,” New Orleans Mayor Mitch Landrieu told reporters.
The US Army Corps of Engineers began to close for the first time the massive new floodgate on the largest storm-surge barrier in the world, at Lake Borgne, east of New Orleans.
Al Jazeera’s Tom Ackerman, reported from New Orleans, where some 50,000 people were set to evacuate from New Orleans.
“People here are, understandably, cautious,” he said. “They are confident that the enhancements to the levee system, particularly the wall that is nearly finished, will protect the entire city of New Orleans from significant damage.”
Earlier on Tuesday, Obama said he had ordered the Federal Emergency Management Agency (FEMA) to be in place more than a week ago to co-ordinate 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 Center, as well as from FEMA in terms of activities on the ground, we’ll be providing continuous updates both local and national level.”
The US National Hurricane Center warned in its 12:00 GMT advisory of a “significant storm surge and freshwater flood threat to the northern Gulf Coast”.
The Miami-based NHC said “preparations to protect life and property should be rushed to completion”.
Alabama’s governor has lifted the mandatory evacuation order for parts of Alabama’s two coastal counties.
Robert Bentley had ordered residents of low-lying coastal areas and flood-prone areas of Mobile and Baldwin counties to evacuate Monday. But he announced on Tuesday that he was revising the order because the National Hurricane Center lifted its hurricane warning for the Alabama coast.
However, Bentley is still recommending that people in low-lying, flood-prone areas move to safer locations because of heavy rains and storm surge forecast at 1.2 to 2.4 metres.