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Hurricane Katrina strengthened into a powerful Category Five hurricane overnight with sustained winds of 160 mph. The National Hurricane Center put out a special advisory on the hurricane's gain in strength just before 8 a.m. EDT. The boost came just hours after Katrina reached Category 4, with wind of 145 mph, as it gathered energy from the warm water of the Gulf of Mexico. According to the National Hurricane Center, a Category Five hurricane causes storm surges generally greater than 18 ft above normal, complete roof failure on many residences and industrial buildings. All shrubs, trees, and signs are blown down. Severe and extensive window and door damage can occur. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline occurs and massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required. This is especially essential in the New Orleans area where most of the city lies below sea level and exists with the help of levees and pumps. To date, only 3 Category Five Hurricanes have made landfall in the United States since records began. + Click for high resolution of image above. Credit: NASA/Jeff Schmaltz, MODIS Land Rapid Response Team Updated August 28, 2005 8:42 a.m. EDT