Scott Kroehle, left, and John Witherspoon board up one of the many majestic Garden District homes in New Orleans

In the southern U.S. city of New Orleans, authorities are ordering all residents to evacuate before the expected arrival of a severe hurricane with winds of 250 kilometers per hour. Hurricane Katrina has intensified over the Gulf of Mexico and is heading for New Orleans, a city of nearly 500,000.

Hurricane Katrina was barely a hurricane when it hit Miami and other parts of southeast Florida on Thursday night. The storm caused extensive flooding there and left nearly 1.5 million people without electrical power.

But as Katrina moved into the Gulf of Mexico, it became one of the strongest hurricanes to ever threaten the United States, becoming a Category 5 on a scale of one to five.

In the coastal state of Louisiana, Governor Kathleen Blanco says there is great concern for the city of New Orleans, which is nearly two meters below sea level and situated on a large lake near the Gulf coast. She spoke on the NBC Today program.

"So, we're more vulnerable than ever before," she said. "With Lake Pontchartrain sitting right next to the city, there's always been the nightmare thought of the lake pouring into the city, which is below sea level. It technically is a bowl, and this bowl could fill up with water."

The mayor of New Orleans ordered mandatory evacuations from the city, except for emergency and other personnel.

President Bush has already declared a federal state of emergency for the state of Louisiana.

New Orleans has not seen a direct hit from a hurricane since a weaker storm, named Betsy, killed 75 people in 1965, and caused $7 billion in damage. The American Red Cross has said a hurricane hitting New Orleans could be the deadliest natural disaster to ever occur in the United States.