Hurricane Gustav charged across the Gulf of Mexico on Sunday as residents fled New Orleans and the National Guard prepared to patrol evacuated neighborhoods in a city still recovering three years after Katrina struck. Gustav dropped from a Category 4 to a Category 3 storm overnight, but forecasters warned it could gain strength from the gulf's warm waters before making landfall as early as Monday. Long before Mayor Ray Nagin's mandatory evacuation order took effect Sunday morning for the city's vulnerable West Bank, residents were already streaming out of New Orleans and other communities along the Gulf Coast. Bumper-to-bumper traffic was reported in nearly every direction out of New Orleans, and on Bourbon Street, where the party seemingly never ends, only stragglers toting luggage were sporadically seen on the sidewalks. Gustav crossed western Cuba on Saturday and has already killed more than 80 people in the Caribbean. It picked up speed upon reaching the gulf and was moving northwest at 17 mph (27 kph) with winds of 120 mph (193 kph), according to the National Hurricane Center's 11 a.m. EDT (1500 GMT) update. Hurricane-force winds extended 50 miles (80 kilometers) from the storm's center.