On the cusp of Hurricane Katrina’s third anniversary, nervous Gulf Coast residents watched Wednesday as a storm threatened to strengthen and crash ashore, testing everything the city has rebuilt.

Forecasters warned that Gustav had the potential to grow into a perilous Category 3 hurricane and approach the Gulf Coast by Monday morning — though cautioned that a storm’s track and intensity are extremely difficult to predict several days in advance.

“We know it’s going to head into the Gulf. After that, we’re not sure where it’s heading,” said Rebecca Waddington, a meteorologist at the National Hurricane Center. “For that reason, everyone in the Gulf needs to be monitoring the storm.”

City officials were taking no chances, and drawing blueprints of how to evacuate the city if necessary. New Orleans plans to institute a mandatory evacuation order should a Category 3 or stronger hurricane be within 72 hours of the city.

At a suburban Lowe’s store, employees said portable generators, gasoline cans, bottled water and batteries were selling briskly. Hotels across south Louisiana reported busy booking business as coastal residents considered their inland refuge options.

Steve Weaver, 82, and his wife Helen stayed for Katrina — and wound up being plucked off the roof of their house and by a Coast Guard helicopter. This time, Weaver said he has no inclination to stay. “Everybody learned a lesson about staying, so the highways will be twice as packed this time,” Weaver said.

Katrina struck New Orleans on Aug. 29, 2005, and its storm surge blasted through the levees that protect the city. Eighty percent of the city was flooded, which set into motion a multibillion-dollar rebuilding program.

Since then, the Army Corps of Engineers has spent billions of dollars to improve the levee system. Though experts say the city and surrounding region are safer from hurricanes, the improved levee protection is incomplete and holes remain.

Floodgates have been installed on drainage canals in New Orleans to cut off storm surge from entering the city, and levees have been raised and in many places strengthened with concrete.

Robert Turner Jr., the regional levee director, said the levee system can handle a storm with the likelihood of occurring every 30 years, what the corps calls a 30-year storm. By comparison, Katrina was a 396-year storm.

“There’s always the possibility if it comes from the right direction, and if it is large enough to create storm surge in the realm of Katrina, that there could be overtopping” of levees, Turner said.

Gustav formed Monday and roared ashore Tuesday as a Category 1 hurricane near the southern Haitian city of Jacmel with top winds near 90 mph, toppling palm trees and flooding the city’s Victorian wooden buildings. It triggered flooding and landslides that killed at least 11 people in the Caribbean. It weakened into a tropical storm and appeared headed for Cuba, though is likely to grow stronger in the coming days by feeding on warm open water.

David Nolan, an associate professor at University of Miami’s Rosenstiel School of Marine and Atmospheric Science, says the uncertainty surrounding the track and strength of Gustav is “very large.” Forecasts show the storm hitting anywhere from the Florida Panhandle to Texas.

Making a comparison to Hurricane Katrina is way too speculative at this early point in the forecast, he said.

“You can’t look at this storm and make any comparisons to Katrina,” he said.

Some New Orleans residents weren’t deciding on their course of action just yet. Just blocks from where a levee breached in the Lakeview neighborhood during Katrina, Lawson “Bunny” Brannan, a construction company owner, was busy renovating his home Wednesday. A wall of Katrina-driven water up to 15 feet deep wiped out the home.

He calmly went about his business, but nonetheless kept a watchful eye on the weather.

“I’m not going to worry about it until I see it in the Gulf, then I’ll make my decisions,” Brannan said.