Station 42040, located at 29°11'03"N 88°12'48"W approximately 64 nautical miles south of Dauphin Island Alabama, reported a significant wave height of 16.91 meters (55.5 feet) at 1100 UTC, August 29, 2005.
Station 42040 is a 3-meter diameter discus hull buoy deployed and operated by National Oceanographic and Atmospheric Administration's National Data Buoy Center (NDBC).
Although 42040 does not measure maximum wave heights, the maximum wave height may be statistically approximated by 1.9 times the significant wave height (World Meteorological Organization, 1998), which would be 32.1 meters (105 feet). At the time of the report, Hurricane Katrina was approximately 73 nautical miles to the west of 42040 with maximum sustained winds of 145 miles per hour (Public Advisory 26A issued by the National Hurricane Center). In addition to the 55-foot report, 42040 reported seas 12 feet or greater for 47 consecutive hours.

The 55-foot report surpasses the previous highest significant wave height reported by an NDBC buoy in the Gulf of Mexico of 15.96 meters (52 feet), also reported by 42040 during Hurricane Ivan in September 2004, and matches the previous highest significant wave height reported by an NDBC buoy of 16.91 meters reported by station 46003 (in the Northeast Pacific Ocean south of the Aleutian Islands) in January 1991.

http://www.ndbc.noaa.gov/hurricanes/katrina/