That night , the storm underwent another period of rapid strengthening , strengthening to a Category 5 hurricane with maximum sustained winds of 175 mph ( 280 km\/h ) by 10:00 am CDT on August 28 . On the night of August 27 , Katrina underwent another episode of rapid deepening , strengthening to a Category 5 hurricane with maximum sustained winds of 175 mph ( 280 km\/h ) by 10:00 am CDT on August 28 .