Hurricane Katrina was a powerful, devastating and historic tropical cyclone that caused 1,392 fatalities and damages estimated at $125 billion in late August 2005, particularly in the city of New Orleans and its surrounding area. It is tied with Hurricane Harvey as being the costliest tropical cyclone in the Atlantic basin. Katrina was the twelfth tropical cyclone, the fifth hurricane, and the third major hurricane of the 2005 Atlantic hurricane season. It was also the fourth-most intense Atlantic hurricane to make landfall in the contiguous United States, gauged by barometric pressure.

Katrina formed on August 23, 2005, with the merger of a tropical wave and the remnants of a tropical depression. After briefly weakening to a tropical storm over south Florida, Katrina entered the Gulf of Mexico on August 26 and rapidly intensified to a Category 5 hurricane before weakening to a Category 3 at its landfall on August 29 near Buras-Triumph, Louisiana.

Eighty percent of New Orleans, as well as large areas in neighboring parishes, were flooded. It is estimated that about 100,000 to 150,000 people remained in the City of New Orleans, despite mandatory evacuation orders. This prompted a massive national and international response effort, including federal, local, and private rescue operations. The largest loss of life was due to flooding caused by engineering flaws in the federally built hurricane protection system, particularly the levees around New Orleans.