NEW ORLEANS (CNN) — Louisiana officials urged the remaining residents of the state’s coastal parishes to head north ahead of Hurricane Gustav, which was sweeping toward the Gulf Coast with 115-mph winds Sunday afternoon.

Highways north were packed with evacuees from Louisiana and Mississippi, many of them headed to shelters inland. Others with medical conditions were flown out or took trains from New Orleans north — efforts that were stepped up after the catastrophe of Hurricane Katrina in 2005.

“It was bumper-to-bumper for about 10 hours trying to get out,” said Roberto Ascencio, a restaurant owner in the New Orleans suburb of Gretna. He said he, his wife and daughter packed their pets and whatever else would fit in their car and headed east on Interstate 10 as Gustav neared.

“It was very light when I left my house. I thought it was going to be a piece of cake,” he said. “As soon as we hit the interstate, it was bumper-to-bumper. It was very very slow moving.”

New Orleans Mayor Ray Nagin ordered a mandatory evacuation and a curfew and warned against looting. And Gov. Bobby Jindal said that while forecasters can’t predict exactly where Gustav will hit, he urged his citizens to “get out of harm’s way.”

“You still have daylight. You still have hours to evacuate,” Jindal said at a Sunday afternoon news conference. “I don’t want people in our coastal communities thinking they’re going to ride out this storm or that they should ride out this storm.”

Meanwhile, three years after his response to Hurricane Katrina received widespread criticism, President Bush canceled plans to attend this week’s Republican National Convention as his administration scrambled to prepare for Gustav’s arrival. The GOP also curtailed its plans for the convention as the storm neared.

In Mississippi, which took the brunt of Katrina’s winds and storm surge three years ago, Gov. Haley Barbour said government agencies were “10 times better prepared” than before — but “that doesn’t mean everything is going to go right,” he said.

“Anybody who thinks everything is going to go perfect just doesn’t know what they’re talking about,” Barbour said.

At 5 p.m. ET, forecasters at the National Hurricane Center projected the storm would strike land southwest of New Orleans sometime Monday. The Category 3 storm was about 215 miles (350 km) southeast of the mouth of the Mississippi River, the National Hurricane Center in Miami reported, and it was moving across the central Gulf of Mexico at 18 mph.

Maps of Gustav’s path show that it could strike southern Louisiana and other areas battered by Hurricanes Katrina and Rita in 2005. Katrina, a Category 3 storm, flooded most of New Orleans, flattened beach towns in Mississippi and killed more than 1,800 people.

Gustav already has been blamed for more than 60 deaths in the Caribbean, including 51 in southwestern Haiti. It struck Haiti, the Dominican Republic, Jamaica and Cuba before crossing into the Gulf of Mexico and growing into a major hurricane.

Forecasters warned that Gustav could grow to a Category 4 hurricane before hitting land.

“We are pleading with the public — please do not remain in St. Bernard Parish for this storm,” said Craig Taffaro, president of a suburban New Orleans parish devastated by Katrina. “We will do everything we can to open the parish immediately after the storm when conditions are safe to do so.”

Nagin said a city-wide curfew would continue until the threat of the storm passes, and he warned that looters would be dealt with harshly.

“Anybody who’s caught looting in the city of New Orleans will go directly to Angola,” he said, referring to Louisiana’s notorious maximum-security prison.

But some, like Louisiana resident Nick Dominque, planned to remain behind. Dominque, 30, told CNN that he had weathered Hurricanes Rita and Katrina and needed to look after his parents in the southern part of the state.

“They think that they can handle any storm that hits, so I’m going to bunker down and stay with them,” he said. “I never thought after Rita that I would try to ride out another storm. But here I am again.”

Homeland Security Secretary Michael Chertoff, who was overseeing evacuation efforts from New Orleans, said most people have chosen to leave, either by car, bus, ambulance or air.

“I was reading in the paper about people who say, ‘No I’m going to stay.’ I think that’s pretty stupid, but I can’t stop that from happening,” he said. “I’m hoping it’s a very small number of people.”

Barbour said mandatory evacuations were ordered for two of the three Mississippi coastal counties, while the third has ordered evacuations only from low-lying areas. In Hancock County, along the Mississippi state line, he said forecasters currently predict storm surges of five to eight feet — but if Gustav shifts just 20 miles eastward, “We could see storm surges of 15-20 feet,” he said.

Bush warned Gulf Coast residents Sunday that a “serious” storm was headed their way and announced plans to head to Texas to meet with emergency workers and evacuees. Both the president and Vice President Dick Cheney canceled plans to attend the Republican convention in St. Paul, Minnesota, and convention events have been cut back sharply as the storm approached.

“This is a time when we have to do away with our party politics and we have to act as Americans,” said Arizona Sen. John McCain, the party’s presidential nominee-in-waiting. The party would suspend most of Monday’s activities “except for those absolutely necessary,” McCain said from St. Louis, Missouri.

Republican National Committee Chairman Mike Duncan said the convention would have to be called to order and its credentials committee report received so delegates can formally nominate McCain and his running mate, Alaska Gov. Sarah Palin.

“I would say that the main information I can give you today is that owing to the fact that the senator has asked us to take our Republican hats off and put our American hats on, tomorrow’s program will be business only, and we’ll refrain from any political rhetoric that would be traditional in an opening session of a convention,” Davis said.