Hurricane Katrina was the eleventh named tropical storm, fourth hurricane, third major hurricane, and first Category 5 hurricane of the 2005 Atlantic hurricane season. It was the third most powerful storm of the season, behind Hurricane Wilma and Hurricane Rita, and the sixth-strongest storm ever recorded in the Atlantic basin. It first made landfall as a Category 1 hurricane just north of Miami, Florida on August 25, 2005, resulting in a dozen deaths in South Florida and spawning several tornadoes which happened not to strike any dwellings. In the Gulf of Mexico it strengthened into a formidable Category 5 hurricane with maximum winds of 175 mph and minimum central pressure of 902 mbar. It weakened considerably as it was approaching land, making its second landfall on the morning of August 29 along the Central Gulf Coast near Buras-Triumph, Louisiana with 125 mph winds and 920 mbar central pressure, a strong Category 3 storm (having just weakened from Category 4 as it was making landfall).

The sheer physical size of Katrina caused devastation far from the eye of the hurricane; it was possibly the largest hurricane of its strength ever recorded, but estimating the size of storms from before the 1960s (the pre-satellite era) is difficult to impossible. On August 29, its storm surge breached the levee system that protected New Orleans from Lake Pontchartrain and the Mississippi River. Most of the city was subsequently flooded mainly by water from the lake. Heavy damage was also inflicted onto the coasts of Mississippi and Alabama, making Katrina the most destructive and costliest natural disaster in the history of the United States and the deadliest since the 1928 Okeechobee Hurricane.

The official combined (direct and indirect) death toll now stands at 1,383, the fourth or fifth highest in U.S. history (behind the Galveston Hurricane of 1900, the Okeechobee Hurricane of 1928, the 1893 Sea Islands Hurricane, and possibly the 1893 Chenier Caminanda Hurricane). As of December 20, 2005, more than 4,000 people remain unaccounted for, so the death toll may still grow. [2] As of November 22, 2005, 1,300 of those missing were either in heavily-damaged areas or were disabled and "feared dead"; if all 1,300 of these were to be confirmed dead, Katrina would surpass the Okeechobee Hurricane and become the second-deadliest in US history and deadliest in over a century. [3]

Over 1.2 million people were under an evacuation order before landfall.[4] In Louisiana, the hurricane's eye made landfall at 6:10am CDT on Monday, August 29. After 11:00 am CDT, several sections of the levee system in New Orleans collapsed. By early September, people were being forcibly evacuated, mostly by bus to neighboring states. More than 1.5 million people were displaced — a humanitarian crisis on a scale unseen in the U.S. since the Great Depression. The damage is estimated to be about $75 billion by the NHC (with other estimates ranging from $40 to $120 billion) [5], at least double the previously most expensive Hurricane Andrew, making Katrina the most expensive natural disaster in U.S. history.

Federal disaster declarations blanketed 90,000 square miles (233,000 km²) of the United States, an area almost as large as the United Kingdom. The hurricane left an estimated three million people without electricity, taking some places several weeks for power to be restored (but faster than the four months originally predicted). On September 3, Homeland Security Secretary Michael Chertoff described the aftermath of Hurricane Katrina as "probably the worst catastrophe, or set of catastrophes" in the country's history, referring to the hurricane itself plus the flooding of New Orleans.

For a timeline of events leading up to Hurricane Katrina through to the aftermath of the hurricane, see Timeline of Hurricane Katrina.