Ernst Rauch

02 Hurricane Katrina: Meteorological aspects

Hurricane Katrina developed out of a low-pressure vortex over the Bahamas on 23 August. As the eleventh tropical cyclone of the 2005 hurricane season, it crossed South Florida in the Miami area as a Category 1 hurricane (measured on the Saffir-Simpson Hurricane Scale).

In the days that followed, Katrina moved over the eastern part of the Gulf of Mexico with a rapid increase in intensity. Over those areas where the water temperature was 1-2°C above the long-term average, the hurricane already reached force 5 on 28 August. This corresponds to wind speeds of approx. 340 km/h in peak gusts.

Shortly before making landfall on 29 August in the state of Louisiana — some 30-50 km east of New Orleans — it weakened to a Category 4 hurricane. An analysis of wind speed data published by the National Hurricane Center in Miami in December 2005 adjusted its strength at landfall again, lowering it even further to Category 3. Upon landfall in Louisiana and when it moved on to the states of Mississippi and Alabama, Katrina caused massive windstorm damage and, initially on a local scale, flood damage due to torrential rain.

Just a few hours after the hurricane vortex had passed over South Louisiana, the levees were breached on Lake Pontchartrain and on an artificial drainage canal. Large parts of New Orleans were flooded. The affected areas lie below sea level in a kind of soup bowl, and there is no natural drainage.

As draining is only possible using pumps or by natural evaporation, it took several weeks to dry out the city. It was not until early December 2005 that important infrastructure installations were back in place and access to the city of New Orleans was completely restored.