By the afternoon of August 28, the storm was large enough that some areas of the Gulf Coast were already experiencing tropical storm-force winds. The center of Katrina was about 180 statute miles (290 km) from the mouth of the Mississippi River, but tropical storm-force winds extended from the center of the storm, and hurricane-force winds extended about 105 miles (170 km) away. 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 deteriorated before an outer eyewall had fully formed, playing an important role in the weakening. In 18 hours, the hurricane's maximum sustained winds decreased from to . However, storm surge remained high at landfall because large waves greater than in height were generated beforehand (with a buoy recording a 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.