Hurricane Katrina was a catastrophic tropical cyclone, that had a long and complex meteorological history, spanning a month from August 8, to September 7, 2005. Katrina's origins can be traced to the mid-level remnants of Tropical Depression Ten, a tropical wave, and an upper tropospheric trough. The tropical depression emerged as a wave off West Africa on August 8, the second wave followed on August 11, while the trough factored into tropical cyclogenesis between August 17 and 23. The mid-level remnants of Tropical Depression Ten merged with the second tropical wave on August 19 while located north of Hispaniola. Subsequent interaction with the trough spurred convective development, resulting in the formation of Tropical Depression Twelve over the Bahamas on August 23. Deep convection soon blossomed and following the development of rainbands the system intensified into a tropical storm. As it strengthened into a hurricane, Katrina made its first landfall in the Miami metropolitan area on August 25.

The flat terrain of the Everglades did little to disrupt the core of Katrina, only leading to slight weakening before the cyclone emerged over the Gulf of Mexico. There, exceptionally favorable environmental conditions consisting of high sea surface temperatures, low wind shear, an upper-level anticyclone, and high ocean heat content fueled two periods of rapid intensification. Punctuated by an eyewall replacement cycle on August 27 that dramatically expanded the hurricane's size, Katrina ultimately reached its peak strength as a Category 5 hurricane on the Saffir–Simpson scale on August 28. Its maximum sustained winds reached 175 mph (280 km/h) and its pressure fell to 902 mbar (hPa; 26.63 inHg), ranking it among the strongest ever recorded in the Gulf of Mexico. As the hurricane turned north toward Louisiana, it steadily weakened but continued to expand in size.

Katrina struck southeastern Louisiana on August 29 as a Category 3 hurricane, resulting in one of the greatest catastrophes in modern times in the United States. A record-breaking storm surge and destructive winds decimated coastal communities of Louisiana and Mississippi. The subsequent collapse of levees in New Orleans led to a prolonged humanitarian crisis with floodwaters persisting for nearly two months. Nearly 1,400 people died directly or indirectly from the hurricane and damage totaled $125 billion. As the hurricane moved inland it quickly weakened, degrading below tropical storm strength over the Ohio River Valley on August 30. It was soon absorbed by a cold front in the region the next day. Thereafter, the remnants progressed northeast as an extratropical cyclone. It briefly deepened and stalled over Quebec, Canada, in early September before resuming its journey northeast. The cyclone ultimately dissipated in its entirety on September 7 near Greenland.

Origins The origins of Hurricane Katrina are complex and involve the interaction of three weather systems: a tropical wave, the remnants of Tropical Depression Ten, and an upper tropospheric trough. On August 8, 2005, a tropical wave emerged over the Atlantic Ocean from the coast of West Africa. A second wave emerged over the Atlantic on August 11. The former system gradually organized and became Tropical Depression Ten on August 13 while located 1,065 mi (1,715 km) east of Barbados. Strong wind shear produced by an upper tropospheric trough over the western Atlantic caused the depression to quickly degrade into a remnant low as it progressed west-northwest. A subsequent convective burst caused its low- and mid-level circulations to detach, with the latter lagging behind. Tropical Depression Ten's remnant vorticity continued west and dissipated near Cuba on August 21. During operational analyses by the National Hurricane Center (NHC), there was uncertainty over the degree of involvement of the remnants of Ten with the eventual formation of Katrina; however, thorough analysis determined them to be two separate systems. The slower-moving mid-level feature was overtaken by the second tropical wave on August 19. This spurred the formation of widespread, disorganized convection north of Puerto Rico. Moving slowly west-northwest, this feature gradually consolidated as it moved north of Hispaniola to a position east of the Turks and Caicos on August 22.

The upper-tropospheric trough draped across the western Atlantic in the week leading to Katrina's formation provided the final factor leading to the hurricane's genesis. Low-level vorticity associated with this feature led to persistent convection over Cuba and Hispaniola from August 17 to 23, with locally enhanced instability allowing for its persistence. The mid-level remnant of Tropical Depression Ten and the second tropical wave interacted with this trough on August 22, triggering further development of convection. A vorticity maximum then developed over Hispaniola and progressed northeast over the Bahamas. The upper-tropospheric trough subsequently propagated west toward Florida, leading to a relaxation in wind shear over the developing cyclone. This spurred further cyclogenesis and the disturbance finally became Tropical Depression Twelve around 12:00 UTC on August 23 while located about 200 mi (325 km) southeast of Nassau.

Initial development and Florida landfall

During its nascent stages, Tropical Depression Twelve moved slowly northwest in response to a subtropical ridge over the central Atlantic. This placed the system over high sea surface temperatures averaging more than above normal. Ocean heat content (OHC) values across the Bahamas averaged 78 kJ cm–2 and increased to 99 kJ cm–2 as the storm approached Florida, providing ample energy for intensification. Convection deepened throughout August 23 into August 24 with some cloud tops reaching . A defined banding feature also began wrapping around the northern side of the depression. Based on aircraft reconnaissance data, the NHC assessed the depression to have intensified into a tropical storm over the central Bahamas by 12:00 UTC on August 23. At this time it was assigned the name Katrina, the eleventh named storm of the annual hurricane season. Katrina turned north-northwest through a weakness in the subtropical ridge as it organized. The center was also pulled north as it gravitated toward convective bursts to the north. Multiple mesovortices were present within these bursts. An upper-level anticyclone became established over the tropical storm during the latter half of August 24, providing Katrina with symmetrical outflow. Banding features subsequently expanded around the cyclone. By August 25, the subtropical ridge moved farther east and ceased being a factor in Katrina's movement. A new ridge along the United States Gulf Coast took its place and caused the tropical cyclone to turn west toward Florida.

Katrina's westward turn on August 25 was accompanied by an intense convective burst with cloud tops colder than . An increase in wind shear kept the storm's structure asymmetric, pushing dry air into its circulation, with the strongest convection displaced to the south and east, but did not become strong enough to significantly disrupt it. The warm waters of the Gulf Stream fueled persistent convection as Katrina approached Florida. An eye-like feature began developing during the latter half of August 25. Katrina intensified into a Category 1 hurricane on the Saffir–Simpson scale (SSHS) around 21:00 UTC. NOAA aircraft reconnaissance recorded surface winds of and Doppler weather radar velocities returned winds of at a height of . The upper-level ridge strengthened slightly as Katrina approached South Florida, causing the hurricane to turn west-southwest. This was contrary to the NHC's forecast at the time which had Katrina maintaining a west-northwest to northwest trajectory across the Florida Peninsula and later the Florida Panhandle. Southerly inflow over the warm coastal waters fueled intense convection and enabled further strengthening.

The hurricane soon developed a well-defined eye about 25 mi (40 km) in diameter and made landfall at 22:30 UTC along the Miami-Dade/Broward County line. At this time, the storm had maximum sustained winds of 80 mph (130 km/h) and a central pressure of 984 mbar (hPa; 29.06 inHg). A weather station in Virginia Key measured sustained winds of , and the NHC office—which went through the center of Katrina's eye—had sustained winds of and a gust to . A minor storm surge occurred along the southeastern Florida coast. While traversing Monroe County, Katrina briefly weakened to a tropical storm before emerging over the Gulf of Mexico around 05:00 UTC on August 26. The low-lying, swampy terrain of the Everglades had little effect on Katrina's structure and its overall appearance improved as it traversed the Peninsula. The lack of terrain-related shear and higher latent heat releases compared to non-swampy terrain enabled this to take place. This phenomenon is called the brown ocean effect. Meteorologists described Katrina as "still...an impressive cyclone."

Rapid intensification and peak strength

Emerging over the Gulf of Mexico around 05:00 UTC on August 26, Katrina quickly regained strength and became a hurricane just one hour later. A strong rainband along the southern side of the storm produced torrential rains and strong winds across the Florida Keys. The Dry Tortugas observed sustained hurricane-force winds, with a peak gust of , late in the day as Katrina moved toward the central Gulf. An expansive upper-level anticyclone overspread the Gulf, producing an environment of low wind shear and ample outflow. Sea surface temperatures averaged across much of the Gulf, providing the hurricane with ample fuel. Furthermore, localized temperatures exceeding were present within two warm-core eddies that broke off the Gulf Loop Current. These warm waters led to increasing heat flux in the northeastern quadrant of the hurricane. Kafatos et al. (2006) hypothesized that high sea surface temperatures in the northeast quadrant were vital in Katrina's intensification. Furthermore, Noel Davidson et al. (2008) hypothesized that an upstream Rossby wave influenced this intensification. With these favorable conditions, Katrina underwent two periods of rapid intensification from August 26 to 28, punctuated by an 18-hour pause on August 27. Of the five primary factors for rapid intensification described by Kaplan and DeMaria (2003), conditions were optimal for four of them.

From 06:00 UTC on August 27 to 12:00 UTC the next day, Katrina's maximum sustained winds increased from 75 mph (120 km/h) to 115 mph (185 km/h), marking its intensification to a Category 3 hurricane on the SSHS. A well-defined eye developed during this time. Significant lightning outbursts occurred in Katrina's eyewall throughout the rapid intensification phase. Throughout the remainder of August 27, Katrina's intensity plateaued as an eyewall replacement cycle took place. Its inner eyewall collapsed as a larger one formed around it and subsequently contracted by the morning of August 28. During this cycle, Katrina's overall size expanded dramatically, nearly doubling by the end of the day, with tropical storm-force winds extended up to 160 mi (260 km) from its center. Upwelling of cold-core eddies occurred in the hurricane's wake, with sea surface temperatures falling by near the Florida Keys.