Overnight on August 29, and into the morning of the next day, Katrina quickly weakened (in terms of maximum sustained winds) as it began to enter another eyewall replacement cycle. The inner eyewall disappered before an outer eyewall had fully formed, which was one of the reasons of its rapid weakening. In 18 hours, the hurricane's maximum sustained winds decreased from 170 mph (280 km/h) to 125 mph (205 km/h). However, storm surges remained high at landfall because large waves greater than 30 feet (9.1 m) in height were generated beforehand (with a buoy recording a 55 ft/16.7 m wave at sea), when Katrina was at Categories 4 and 5 on the Saffir-Simpson Hurricane Scale. The waves then combined with the storm surge of the large Category 3 hurricane.