(CNN) — Hurricane Gustav weakened slightly in the early afternoon hours Sunday but still was predicted to be a major hurricane when it smack into the Gulf Coast sometime Monday.

Hurricance Gustav was barreling toward southern Lousiana, sending Hurricane Katrina survivors fleeing and prompting President Bush to alter his plans and forcing New Orleans Mayor Ray Nagin to institute a curfew.

As of 2 p.m. ET, the eye of the Category 3 storm was about 270 miles (435 km) southeast of the mouth of the Mississippi River, the National Hurricane Center in Miami said.

The storm was moving at 17 mph across the central Gulf of Mexico on Sunday and is expected to reach the U.S. Gulf Coast on Monday, forecasters said.

The storm had maximum winds of 115 mph with higher gusts, making it a Category 3 hurricane, forecasters said. The storm could increase to Category 4 storm, with winds of 131 to 155 mph, forecasters said.

Maps of Gustav’s path show that it could strike southern Louisiana and other areas battered by Hurricane Katrina in 2005.

That storm hit the area at a Category 3, caused severe flooding and killed more than 1,800 people.

The White House said Sunday that Bush and Vice President Dick Cheney will not attend the Republican National Convention Monday as planned because of Hurricane Gustav, the White House said Sunday.

The president had been scheduled to address delegates in St. Paul, Minnesota, Monday night.

Bush Sunday warned residents of the Gulf Coast that a “serious” storm was headed their way. He said he planned to head to Texas to meet with emergency workers and evacuees.

“This storm is dangerous,” Bush warned, urging resident to heed calls by state to evacuate.

Republican officials are monitoring the storm and said it could cause “substantial” changes to the convention program.

Presidential candidate John McCain hinted at the potential changes to reporters gathered in Pearl, Mississippi, Sunday.

“We must redirect our efforts from the really celebratory event of the nomination of president and vice president of our party to acting as all Americans,” McCain said. “We’ll change our program and I’ll be announcing details of it in the next few hours.”

Gustav killed at least 51 people in southwestern Haiti and eight in neighboring Dominican Republic last week before moving on to Cuba. It was in the Caribbean Friday and intensified just before it hit Cuba.

Roadways heading out of Lousiana were clogged with cars Sunday, CNN personnel observed.

Louisiana resident Nick Dominque, 30, said he would stay behind to look after his parents who live in southern part of the state.

Dominque was one of the many I-Reporters who contacted CNN. He said he had weathered Hurricanes Rita and Katrina.

“This is a very dangerous storm and we are all hoping for the best,” said Dominque. “I have older parents, and living in South Louisiana, they think that they can handle any storm that hits. So I’m going to bunker down and stay with them. I never thought after Rita that I would try to ride out another storm. But here I am again.”

Aaron Broussard, president of Louisiana’s Jefferson Parish called for a mandatory evacuation of the area Sunday morning. The evacuation will start at noon Sunday, the same time New Orleans officials scheduled the evacuation of that city.

As he reiterated his call urging New Orleanians to leave, Nagin said Sunday the city imposed a “dusk to dawn” curfew and in the afternoon plans to cease its efforts to help people leave the city.

Nagin said the 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 (Louisiana State Penitentiary    State),” he said.

Local and federal officials — eager to prove they are ready for the storm — urged residents across the region to flee. Many of those residents obeyed, moving north by the tens of thousands, according to Louisiana Gov. Bobby Jindal.

Charter flights, paid for with federal funds, carried thousands of evacuees to other southern cities, including Knoxville, Nashville and Memphis in Tennessee.

Late Saturday night, several hundred New Orleans’ citizens stepped off a jet at the Tennessee Air National Guard hangar at Nashville’s airport to find an organized registration process and buses ready to take them to temporary shelters, part of a detailed plan developed in response to criticism in the aftermath of Katrina’s chaos.

Homeland Security Secretary Michael Chertoff said authorities are responding to Hurricane Gustav better than they reacted to Hurricane Katrina three years ago.

Speaking to reporters outside Andrews Air Force Base in Maryland on Sunday before he headed to the Gulf Coast, Chertoff said he has been pleased with the evacuation process, which started a full day ahead of when evacuations began during Katrina.

In Mississippi, which was also badly damaged by Katrina, Gov. Haley Barbour said his state would cooperate with Louisiana’s “contraflow” plan, so that Louisiana evacuees and those in low-lying coastal areas of Mississippi could flee northward.