New Orleans Mayor C. Ray Nagin on Saturday night ordered a mandatory evacuation of his city ahead of Hurricane Gustav, which swelled from an already deadly tropical storm into a monster depression with winds of more than 150 mph.

“This is the real deal, not a test,” Nagin said as he issued the order, effective at 8 a.m. Central time today for low-lying areas and noon citywide. He warned residents that staying would be “one of the biggest mistakes of your life.”

Forecasters warned that it was still too soon to say whether New Orleans would take a direct hit from Gustav, but the storm’s threat, coming three years after Hurricanes Katrina and Rita devastated a broad swath of the Gulf Coast, drew a healthy amount of respect from city, state and federal officials.

Gustav has already killed more than 80 people in the Caribbean. On Saturday, it slammed into western Cuba, knocking out power in Havana. The Cuban government said that it had moved at least 300,000 people.