Extreme winds associated with a collapsing core on the Mobile waterfront during the landfall of Hurricane Katrina

Hurricane Katrina struck the northern Gulf Coast on 29 August 2005 with the third lowest pressure on record for a landfalling U.S. hurricane. The core of the storm moved inland over southeast Louisiana and southern Mississippi, producing catastrophic damage in these areas. However, very strong squalls in the northeastern quadrant of the storm also moved inland across coastal Alabama. A very heavy squall moved across the Mobile Alabama waterfront at the head of Mobile Bay shortly after 1300 UTC on 29 August 2005. This squall was within one of Katrina's spiral bands and produced extremely strong winds which dislodged a large floating oil drilling platform from its moorings on the east side of the Mobile River. This floating platform was then blown across the river where it collided with another ship before eventually becoming lodged against a large suspension bridge over the Mobile River (see figure, courtesy of (see figure, courtesy of Mike Kittrell/Mobile Press-Register). Eyewitnesses indicate that the platform broke free during the squall. A nearby U.S. Navy ship located directly across the river reported an unconfirmed wind gust of 55 m s^-1 (122 mph) measured on its anemometer about the time the platform broke loose. Landfalling tropical cyclones often contain collapsing cores of heavy precipitation within their eyewalls. Several papers indicate the likely linkage between downburst-like features and strong wind gusts in hurricane eyewalls; however, very few surface-based measurements of winds associated with these features actually exist. In addition, collapsing core-type features are also observed with heavy convection within tropical cyclone rainbands. Doppler radar analysis indicates the squall along the Mobile waterfront was moving rapidly toward the northwest at 42-44 m s^-1 (94-99 mph). Reflectivity cross sections indicate that the squall contained a >50 dBZ collapsing core which reached the surface about the time the squall moved across the moored platform's location. Doppler radar base velocity estimates, corrected for beam azimuth-wind direction differences via a technique by Lee et al., 1999, indicate that winds were estimated to be well in excess of 54 m s^-1 (120 mph) at an elevation as low as 240 m (790 ft) in the vicinity of the platform with the passage of this squall. Momentum from this slightly-elevated wind maximum was likely diverted toward the surface during the passage of the very heavy collapsing precipitation core, thus providing a favorable situation for very strong near-surface wind gusts at the time the platform broke loose. Although a record high tide of approximately 3.7 m (12 ft) occurred along the Mobile waterfront during Hurricane Katrina's landfall, the platform broke loose several hours before the highest water arrived. In addition, large accommodation modules, removed from the platform during renovation, tightly stacked weighing several tons each and situated above the level of peak water, were moved around within the shipyard where the platform was originally moored, a further testament to the extreme winds which affected this location.