NEW ORLEANS (AP) — On the eve of Hurricane Katrina’s third anniversary, a nervous New Orleans watched Wednesday as another storm threatened to test everything the city has rebuilt, and officials made preliminary plans to evacuate people, pets and hospitals in an attempt to avoid a Katrina-style chaos.

Forecasters warned that Gustav could grow into a dangerous Category 3 hurricane in the next several days and hit somewhere along a swath of the Gulf Coast from the Florida Panhandle to Texas — with New Orleans smack in the middle.

‘‘I’m panicking,’’ said Evelyn Fuselier of Chalmette, whose home was submerged in 14 feet of floodwater when Katrina hit. Fuselier said she’s been back in her home one year this month, and called watching Gustav swirl toward the Gulf of Mexico indescribable. ‘‘I keep thinking, ’Did the Corps fix the levees?,’ ’Is my house going to flood again?’ … ’Am I going to have to go through all this again?’’’

Taking no chances, city officials began preliminary planning to evacuate and lock down the city in hopes of avoiding the catastrophe that followed the 2005 storm. Mayor Ray Nagin planned to leave the Democratic National Convention in Denver to return home for the preparations, as did U.S. Sen. Mary Landrieu. Gov. Bobby Jindal declared a state of emergency to lay the groundwork for federal assistance, and put 3,000 National Guard troops on standby.

If a Category 3 or stronger hurricane comes within 60 hours of the city, New Orleans plans to institute a mandatory evacuation order. Unlike Katrina, there will be no massive shelter at the Superdome, a plan designed to encourage residents to leave. Instead, the state has arranged for buses and trains to take people to safety.

It was unclear what would happen to stragglers. Jerry Sneed, the city’s emergency preparedness director, said officials are ready to move about 30,000 people. Nearly 8,000 people had signed up for transportation help by late Wednesday.

At a suburban Lowe’s store, employees said portable generators, gasoline cans, bottled water and batteries were selling briskly. Hotels across south Louisiana reported taking many reservations as coastal residents looked inland for possible refuge.

Steve Weaver, 82, and his wife stayed for Katrina — and were plucked off the roof of their house by a Coast Guard helicopter. This time, Weaver has no inclination to ride out the storm.

‘‘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.

Though pockets of the New Orleans are well on the way to recovery, many neighborhoods have struggled to recover. Many residents still live in temporary trailers, and shuttered homes still bear the ’X’ that was painted to help rescue teams looking for the dead.

Many people never returned, and the city’s population, around 310,000 people, is roughly two-thirds what it was before the storm, though various estimates vary wildly.

Since the storm, the Army Corps of Engineers has spent billions of dollars to improve the levee system, but because of two quiet hurricane seasons, the flood walls have never been tested.

Floodgates have been installed on drainage canals to stop any 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.

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 buildings.

The storm triggered flooding and landslides that killed 23 people in the Caribbean. It weakened into a tropical storm and appeared headed for Cuba, though it is likely to grow stronger in the coming days by drawing energy from warm open water.

Scientists cautioned that the storm’s track and intensity were difficult to predict several days in advance.

But in New Orleans, there was little else to do except prepare as if it were Katrina. The Louisiana Society for the Prevention of Cruelty to Animals was set to begin moving animals inland to shelters in Baton Rouge on Thursday, and more would go to Texas shelters on Friday and Saturday.

‘‘We definitely don’t want to wait until Saturday or Sunday to decide what to do,’’ said Ana Zorrilla, director of the pet-rescue group.

In Grand Isle, tractor loads of dirt and clay mud were being hauled in to fill portions of the levee system damaged by Hurricane Katrina, said Grand Isle Mayor David Camardelle. The coastal community south of New Orleans historically is one of the first to evacuate when tropical weather threatens and was hard-hit by Katrina.

‘‘I couldn’t sleep last night,’’ Carmardelle said. ‘‘We just came back from so much.’’

Emergency preparations also were under way along Mississippi’s coast. The eye of Hurricane Katrina pushed ashore near the small towns of Waveland and Bay St. Louis, Miss., and along the 70-mile coastline, roughly 65,000 homes were destroyed, and thousands of businesses and hulking casino barges were wiped out.

‘‘We don’t need anything of this magnitude to come here,’’ said Biloxi Mayor A. J. Holloway. ‘‘Katrina just devastated us.’’

The oil market also reacted to the threat. Oil prices jumped above $119 a barrel as workers began to evacuate from the offshore rigs responsible for a quarter of U.S. crude production. Any damage to the oil infrastructure or Gulf Coast refineries could send U.S. pump prices spiking, possibly before the busy Labor Day weekend.

‘‘A bad storm churning in the Gulf could be a nightmare scenario,’’ said Phil Flynn, an analyst at Alaron Trading Corp. in Chicago. ‘‘We might see oil prices spike $5 to $8 if it really rips into platforms.’’

Many residents hadn’t yet made a decision about leaving. Lawson ‘‘Sonny’’ Brannan, a construction company owner, was busy renovating a client’s home Wednesday, just blocks from where a levee was breached in the Lakeview neighborhood. A wall of water up to 15 feet deep wiped out the home.

Brannan 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,’’ he said. ‘‘Then I’ll make my decisions.’’