An evacuation order for New Orleans is likely to go into effect Saturday as Hurricane Gustav approaches the Gulf Coast.

Meanwhile, Mississippi officials have told coastal residents hit by Hurricane Katrina three years ago to evacuate this weekend, and Grand Isle, La., residents also are under a voluntary evacuation order. Texas has put its National Guard on standby.

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New Orleans Mayor Ray Nagin said he would order a mandatory evacuation of the city if forecasters predict a Category 3 strike, or possibly even a Category 2, within 72 hours.

Gustav poses the biggest threat to New Orleans since deadly Katrina in 2005.

Forecasters said Hurricane Gustav is expected to cross into the Gulf of Mexico by Sunday.

That's when it could grow to a Category 3, with winds in excess of 111 mph.

Gustav is already blamed for killing at least 71 people in the Caribbean.

Coast Prepares For Worst

During a press conference Thursday in Baton Rouge, La., state and federal heads addressed the serious nature of the storm and the worst-case scenario. Jindal said residents cannot simply wait for Gustav's destruction, urging them to "take this storm seriously." (Video )

Hospitals in New Orleans and elsewhere along the Gulf Coast were anxiously preparing for the storm, looking to forgo a repeat of the aftermath of Hurricane Katrina.

They've canceled elective surgery and stocked up on food and other supplies. And the most fragile patients are already being moved.

At least 140 patients died in hospitals and nursing homes after Katrina made landfall three years ago.

And further away, communities are getting ready for the storm evacuees that shocked the region after Katrina.

On the three-year anniversary of Hurricane Katrina and on the heels of Gustav threatening U.S. landfall, the Arkansas Department of Emergency Management penned disaster plans.

Dave Maxwell, director of the Arkansas Department of Emergency Management, said Fort Smith, Ark., would be able to assist 4,000 Louisiana evacuees. He said the agreement was recently made with state officials, reported TV station KHBS.

During Katrina, more than 12,000 evacuees came to Fort Smith, which presented a flurry of logistical and resource issues. Maxwell said the lessons from Katrina and advance warning of Gustav has neighboring states fine-tuning their plans.

"We know better what to expect this time," he said. "They know better how to evacuate."

National Aid Already En Route

Chertoff, of the Federal Emergency Management Agency, said Thursday that the organization was already working, but said, "Ultimately, success of how we weather the storm lies in the hands of individual citizens." (Video)

And as residents pack their belongings and look to the skies, others were heading to the coast.

The American Red Cross was checking on shelters, deploying trucks that could deliver food, and shipping cots, blankets and hygiene kits into the region.

Tropical Storm Hanna Distracts Little From Gustav

As Gustav soaks up the spotlight, the National Hurricane Center said Tropical Storm Hanna formed northeast of the northern Leeward Islands in the Atlantic, but was a very disorganized weather pattern.

Hanna, the eighth tropical storm of the Atlantic season, had top sustained winds near 50 mph Friday morning.

Hanna could produce rainfall of 1 to 2 inches across parts of the Leeward Islands. It's too soon to say if Hanna will affect the United States.