Meteorological history of Hurricane Katrina
Hurricane Katrina was an extremely destructive Category 5 hurricane that affected the majority of the Gulf Coast. Its damaging trek began on August 23, 2005, when it originated as Tropical Depression Twelve near the Bahamas. The next day, the tropical depression strengthened to a tropical storm, and was named "Katrina"; it proceeded to make landfall on the southern tip of the U.S. state of Florida as a minimal hurricane. In passing across Florida, Katrina briefly weakened below hurricane strength, but it restrengthened once it moved back over water. After passing over Florida, the warm waters  of the Gulf of Mexico allowed it to rapidly intensify to the sixth-strongest Atlantic hurricane in recorded history. Afterward, Katrina made landfall as a Category 3 storm near Buras-Triumph, Louisiana, and once more near the Mississippi/Louisiana border. Katrina progressed northward through the central United States and finally dissipated near the Great Lakes on August 31, when it was absorbed by a cold front. Formation. On August 8, 2005, a tropical wave moved off the coast of Africa and started to move westward across the Atlantic Ocean, before it started to show signs of convective organization on August  11. Over the next couple of days, the system developed further, before it was classified as Tropical Depression Ten by the United States National Hurricane Centre during August 13, while it was located about to the east of the Bahamas. However, conditions surrounding the depression quickly became unfavorable, as a sharp and deep upper-level trough of low pressure developed to the west of the system over the Western Atlantic Ocean and the Bahamas. This upper-level trough caused the system to become sheared, with its low-level circulation center displaced from its deep atmospheric convection. As a result, the  depression degenerated into a non-convective remnant low and advisories from the NHC were discontinued during August 14. Over the next few days, the depression's remnant low-level circulation center continued to weaken, as it moved westward and eventually dissipated on August 21, in the vicinity of Cuba. However, the system's mid-level center had split from the low-level center, which lagged behind and passed to the north of the Leeward Islands during August 18, before it merged with a westward-moving tropical wave that had left Africa on August 11, during August 19. This produced a  large area of disturbed weather to the north of Puerto Rico, which slowly moved north-westward over the next few days to the north of Hispaniola. During August 22, the system started to consolidate just to the east of the Turks and Caicos Islands, while the upper-level trough started to weaken as it moved westwards towards Florida which caused the vertical wind shear to relax. During the following day, a reconnaissance aircraft from the United States Air Force flew into the area of disturbed weather, where it found a broad but well-defined low-level wind field. As a  result, the NHC announced that a tropical depression had formed near or over Long Island in the Bahamas and that they would initiate advisories on the system at 21:00 UTC (17:00 EDT) on August 23. However, there was some internal debate as to if this system was a continuation of Tropical Depression Ten or a brand new tropical depression. After they had reviewed satellite imagery and rawinsonde data from the previous week or so, the NHC decided to treat the system as a new tropical depression and labelled it as Tropical Depression Twelve. After being designated as Tropical Depression 12 , the system moved north-westwards through the western Bahamas, towards a weakness in the subtropical ridge of high pressure near Bermuda. During August 24, NHC reported that the depression had become a tropical storm and named it Katrina, as atmospheric convection increased within its eastern semi-circle. During that day, as Katrina developed an inner core and evolved into a deeper tropical cyclone, it came under the influence of a strengthening mid-upper level ridge of high pressure over the northern Gulf of Mexico and the southern United States. This caused Katrina to turn more westwards and start moving towards  Southern Florida. First Landfall. Initially, Katrina moved northwestward straight through The Bahamas within a weakness in the lower tropospheric subtropical ridge. However, as the storm strengthened, developed an inner core, and became a deeper cyclone, it began to turn westward because of a strengthening middle to upper tropospheric ridge over the northern Gulf of Mexico and Southern United States. Katrina then turned just south of due west as it approached South Florida. A burst of convection allowed Katrina to become the fifth hurricane of the 2005 season on August 25, only two hours before it made landfall around 6:30 p.m.  EST (2230 UTC) between Hallandale Beach and Aventura, Florida. Katrina struck the peninsula with winds, and had a well-defined eye on NEXRAD weather radar, which remained intact throughout its passage over Florida. Parts of the Florida Keys experienced tropical storm winds throughout August 26, with the Dry Tortugas briefly experiencing hurricane-force winds. Gulf of Mexico. The initial United States National Hurricane Center forecasts predicted that Katrina would begin turning northward after landfall, eventually to hit the Florida Panhandle approximately three to four days later. Katrina, however, continued a westerly and west-southwesterly track, which eventually shifted the forecast  track westward to New Orleans. Katrina briefly weakened to a tropical storm over land, but immediately after the storm entered the Gulf of Mexico, the low wind shear, good upper-level outflow, and the warm sea surface temperatures of the Gulf Loop Current caused the storm to intensify rapidly. On August 27, the storm was upgraded to Category 3 intensity, becoming the third major hurricane of the season. An eyewall replacement cycle disrupted the intensification of maximum winds for about 18 hours, but almost doubled the radius of the storm. A second period of rapid intensification started by 7:00 p.m.  CDT on August 27, and by 12:40 a.m. CDT on August 28, Katrina was upgraded to a Category 4 hurricane with maximum sustained winds of . It became a Category 5 (the first in the Gulf of Mexico since Hurricane Allen 25 years prior) storm by 7:00 a.m. CDT, twelve hours after the beginning of the second round of rapid intensification, and reached its peak intensity at 1:00 p.m. CDT with maximum sustained winds of , gusts of and a central pressure of 902 millibars. By the afternoon of August 28, the storm was large enough that some areas of the Gulf  Coast were already experiencing tropical storm-force winds. The center of Katrina was about 180 statute miles (290 km) from the mouth of the Mississippi River, but tropical storm-force winds extended from the center of the storm, and hurricane-force winds extended about 105 miles (170 km) away. Overnight on August 29, and into the morning of the next day, Katrina quickly weakened (in terms of maximum sustained winds) as it began to enter another eyewall replacement cycle. The inner eyewall deteriorated before an outer eyewall had fully formed, playing an important role in the weakening. In 18 hours , the hurricane's maximum sustained winds decreased from to . However, storm surge remained high at landfall because large waves greater than in height were generated beforehand (with a buoy recording a wave at sea), when Katrina was at Categories 4 and 5 on the Saffir–Simpson hurricane scale. The waves then combined with the storm surge of the large Category 3 hurricane. Second and third landfalls. Katrina made its second landfall at 6:10 a.m. CDT on August 29 as a Category 3 hurricane with sustained winds of near Buras-Triumph, Louisiana. Because Katrina had just weakened from Category 4 and  due to the shape of the coastline, sustained Category 4 winds likely existed on land while the eye was over water. At landfall, hurricane-force winds extended from the center, the storm's pressure was , and its forward speed was . As it made its way up the eastern Louisiana coastline, most communities in Plaquemines, St. Bernard Parish, and Slidell in St. Tammany Parish were severely damaged by storm surge and the strong winds of the eyewall, which also grazed eastern New Orleans, causing in excess of $1 billion worth of damage to the city (see Effect of Hurricane Katrina  on New Orleans). Original estimates indicated that Katrina had made this landfall as a Category 4 hurricane, with winds; however, as indicated above, the storm weakened just before landfall to Category 3 intensity. The reasons for this weakening are not completely understood; while the eyewall replacement cycle played a part, slightly increasing shear, dropping sea-surface temperatures, dry air on the western semicircle of the storm, and interaction with the continental landmass also may have played a role in weakening the cyclone. This follows the trend of previous strong cyclones in the Gulf of Mexico: most cyclones with minimum central  pressures of or less have weakened over the 12 hours before making landfall in the Gulf Coast of the United States. A few hours later, after weakening slightly, Katrina made its third landfall near the Louisiana–Mississippi border with sustained winds and pressure, still at Category 3 intensity. Its minimum pressure at its second landfall was , making Katrina the fourth-strongest hurricane on record to make landfall on the United States, behind Hurricane Michael's 919-millibar (27.138 inHg) reading, Hurricane Camille's reading in 1969, and the 1935 Labor Day Hurricane's record. Because the storm was so large , highly destructive eyewall winds and the strong northeastern quadrant of the storm pushed record storm surges onshore, smashing the entire Mississippi Gulf Coast, including towns in Mississippi such as Waveland, Bay St. Louis, Pass Christian, Long Beach, Gulfport, Biloxi, Ocean Springs, Gautier and Pascagoula, and, in Alabama, Bayou La Batre. The surges peaked at in Bay St. Louis, Mississippi, and at as far away as Mobile, Alabama, which saw its highest storm surge since 1917. Storm surge was particularly high due to the hydrology of the region, the hurricane's extreme size, and the fact that it weakened only shortly  before landfall. As Katrina moved inland diagonally over Mississippi, high winds cut a swath of damage that affected almost the entire state. Demise. Katrina maintained hurricane strength well into Mississippi, but weakened thereafter, losing hurricane strength more than inland, near Meridian, Mississippi. It was downgraded to a tropical depression near Clarksville, Tennessee and broke in half. One half continued to race northward, affecting the Central United States along its path, and was last distinguishable in the eastern Great Lakes region on August 31. On August 31, Katrina merged with a frontal boundary and became a powerful extratropical low, causing of  rain in 12 hours, as well as gale-force wind gusts from in southeastern Quebec and northern New Brunswick. In the region of Saguenay and Côte-Nord, rain caused breakdowns and failure in roads. The Côte-Nord region was isolated from the rest of Quebec for at least 1 week. The other half of Katrina broke off in the eastern part of the Appalachians, primarily leading to a significant tornado outbreak in the area from central Georgia to central Pennsylvania, killing two people and causing millions of dollars in additional damage. At 11:00 p.m. EDT on August 31, the center  of the remnant low of what was Katrina had been completely absorbed by a frontal boundary in southeastern Canada, with no discernible circulation.