NEW ORLEANS, Louisiana (CNN) -- Emergency officials in New Orleans braced for a potentially catastrophic blow on Sunday as Hurricane Katrina swept toward the Gulf Coast -- and the city -- with maximum sustained winds near 175 mph.
Mayor Ray Nagin declared a state of emergency on Sunday and ordered a mandatory evacuation of the city the Category 5 storm approached. Forecasters said the storm surge could reach 28 feet.
"We are facing a storm that most of us have feared," Nagin said. "I do not want to create panic, but I do want the citizens to understand that this is very serious and it's of the highest nature. About 70 percent of New Orleans is below sea level, and is protected by a series of levies. The storm surge most likely will top our levee system."
About 485,000 people live in the city, and many began evacuating before sunrise. Louisiana Gov. Kathleen Blanco said that westbound traffic was heavy and that the state police was urging people to travel to the north or east.
Nagin said the city could expect complete loss of electricity and water services once the storm had done its work and that the Superdome, the city's main shelter, "is not going to be a very comfortable place at some point in time."
He said people who must stay in the shelter should bring enough food, water and supplies to last several days.
For years, forecasters have warned of the nightmare scenario a big storm could bring to New Orleans, a bowl of a city that's up to 10 feet below sea level in spots and dependent on a network of levees, canals and pumps to keep dry.
It's built between the half-mile-wide Mississippi River and Lake Pontchartrain, half the size of the state of Rhode Island. Estimates have been made of tens of thousands of deaths from flooding that could overrun the levees and turn New Orleans into a 30-foot-deep toxic lake filled with chemicals and petroleum from refineries, and waste from ruined septic systems.
Katrina was a Category 1 storm with 80 mph wind when it hit South Florida with a soggy punch Thursday that flooded neighborhoods and left nine people dead. As much as 18 inches of rain fell in some areas, flooding streets and homes. Katrina reformed over the warm waters of the Gulf Mexico, strengthening rapidly over the warm water.
NHC forecaster Ed Rappaport said Katrina's strength could fluctuate before it reaches shore but noted the difference between a high Category 4 and a low Category 5 was practically inconsequential.