President Barack Obama added his voice to those of local officials urging residents to hunker down or evacuate if told to do so. "Now's not the time to tempt fate," he said in brief comments. "Listen to your local officials and follow their directions, including if they tell you to evacuate."
At 12:20 p.m. ET, Isaac had 75 mph sustained winds, a mile above the speed needed for a Category 1 hurricane.
National Hurricane Center Director Rick Knabb said "rain bands will become more frequent and more potent" along the northern Gulf of Mexico coast as the day goes on.
Isaac is very wide as storms go, with tropical storm-force winds stretching 205 miles from its center.
Its size and slow motion, Knabb said, will make for a large storm surge, especially in southeast Louisiana where surges up to 12 feet are predicted.
The fact that a tropical storm's winds move counterclockwise will make matters worse, especially for New Orleans if Isaac makes landfall to the west of the city as some models suggest.
"That counterclockwise direction is really a big problem," NBC meteorologist Al Roker said Tuesday on TODAY. "As it continues to bring in those winds from the southeast it's going to be piling water up."
Rainfall of 7-14 inches across the coast as well as inland is likely, and a few places could even see 20 inches, Knabb said.
Residents should expect "a lot of hazards to contend with, even isolated tornadoes" are possible Tuesday and into Wednesday, Knabb said.
At 11 a.m. ET, the center of the storm was located about 80 miles south-southeast of the mouth of the Mississippi River.
It was moving to the northwest at 10 mph and could impact New Orleans seven years to the day that Hurricane Katrina struck on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage.
Click here for the latest information from the National Hurricane Center.
Click here for our special coverage on the storm.