Meteorological history of Hurricane Katrina

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.{{sfn|Richard et al.|2005|p=1}} On August 8, 2005, a tropical wave emerged over the Atlantic Ocean from the coast of West Africa.{{cite report|first=Jack L.|last=Beven|publisher=National Hurricane Center|date=January 17, 2006|accessdate=December 9, 2021|title=Tropical Depression Ten|series=Abbreviated Tropical Cyclone Report|url=https://www.nhc.noaa.gov/data/tcr/AL102005_Ten.pdf}} A second wave emerged over the Atlantic on August 11.{{sfn|Richard et al.|2005|p=1}} 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.{{sfn|Richard et al.|2005|p=1}} 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.{{sfn|Richard et al.|2005|p=1}}{{#tag:ref|The National Hurricane Center has specific rules regarding the reformation of tropical cyclones. When a former tropical cyclone reorganizes sufficiently to be reclassified as a tropical cyclone and is still within its basin of origin, its original number and name are kept upon the re-issuance of advisories. This occurred with Hurricane Ivan in 2004 which maintained a distinct low-level circulation after becoming extratropical and later regenerating into a tropical storm over the Gulf of Mexico. If a system crosses out of its basin of origin or is significantly influenced by other systems, it is reclassified as a separate cyclone. This is the case with Tropical Depression Ten and Hurricane Katrina.{{cite web|type=Report|agency=Office of the Federal Coordinator for Meteorology|publisher=United States Department of Commerce|date=May 2021|accessdate=November 26, 2021|title=National Hurricane Operations Plan|url=https://www.icams-portal.gov/resources/ofcm/nhop/2021_full_nhop_change_2.pdf|pages=3–5}}{{cite report|first=Stacy R.|last=Stewart|publisher=National Hurricane Center|date=August 23, 2005|accessdate=November 26, 2021|title=Tropical Depression Twelve Discussion Number 1|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.001.shtml}}|group=nb}} 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.{{sfn|Richard et al.|2005|p=1}}

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.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|pp=3907–3908}} 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.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3908}} 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.{{sfn|Richard et al.|2005|p=1}}

File:Katrina Florida landfall.jpg

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 {{convert|1|C-change|abbr=on|disp=flip}} above normal.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3912}} 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.{{cite report|first1=Kevin|last1=Law|series=Recent Hurricane Research - Climate, Dynamics, and Societal Impacts|publisher=Marshall University|year=2011|title=The Impact of Oceanic Heat Content on the Rapid Intensification of Atlantic Hurricanes|issue=17|pages=331–354}}{{rp|339}}{{#tag:ref|According to Shay et al. (2000), OHC values of 50 kJ cm^–2 or greater support the intensificaton of hurricanes.{{cite report|first1=John|last1=Lillibridge|first2=Nick|last2=Shay|first3=Mark|last3=DeMaria|first4=Gustavo|last4=Goni|first5=Michelle|last5=Mainelli|first6=Remko|last6=Scharroo|first7=Lamar|last7=Russell|publisher=National Oceanic and Atmospheric Administration|accessdate=August 22, 2023|title=Hurricane Intensity Forecasting at NOAA Using Envisat Altimetry|url=https://www.star.nesdis.noaa.gov/pub/socd/lsa/johnl/envisat/montreux/montreux_nhc_v4.pdf}}{{Dead link|date=February 2025 |bot=InternetArchiveBot |fix-attempted=yes }}|group=nb}} Convection deepened throughout August 23 into August 24 with some cloud tops reaching {{convert|-80|C|F|abbr=on|disp=flip}}.{{cite report|first=Richard D.|last=Knabb|publisher=National Hurricane Center|date=August 24, 2005|accessdate=November 25, 2021|title=Tropical Depression Twelve Discussion Number 2|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.002.shtml|type=Technical Discussion}} 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.{{sfn|Richard et al.|2005|p=2}} 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.{{cite report|first=Stacy R.|last=Stewart|publisher=National Hurricane Center|date=August 24, 2005|accessdate=November 25, 2021|title=Tropical Storm Katrina Discussion Number 4|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.004.shtml|type=Technical Discussion}} Banding features subsequently expanded around the cyclone.{{cite report|first=Stacy R.|last=Stewart|publisher=National Hurricane Center|date=August 24, 2005|accessdate=December 27, 2021|title=Tropical Storm Katrina Discussion Number 5|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.005.shtml|type=Technical Discussion}} 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.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3913}}

Katrina's westward turn on August 25 was accompanied by an intense convective burst with cloud tops colder than {{convert|-80|C|F|abbr=on|disp=flip}}.{{sfn|Richard et al.|2005|p=2}}{{cite report|first=Jack L.|last=Beven|publisher=National Hurricane Center|date=August 25, 2005|accessdate=December 27, 2021|title=Tropical Storm Katrina Discussion Number 7|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.007.shtml|type=Technical Discussion}} 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,{{sfn|Richard et al.|2005|p=2}} but did not become strong enough to significantly disrupt it.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3913}} 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.{{cite report|first=Stacy R.|last=Stewart|publisher=National Hurricane Center|date=August 25, 2005|accessdate=December 27, 2021|title=Tropical Storm Katrina Discussion Number 8|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.008.shtml|type=Technical Discussion}} Katrina intensified into a Category 1 hurricane on the Saffir–Simpson scale (SSHS){{#tag:ref|The Saffir–Simpson scale as used in 2005 consisted of sustained winds, central pressures, and expected storm surge values. This scale was modified in 2010 to only encompass winds, now being called the Saffir–Simpson hurricane wind scale.{{cite report|publisher=Office of the Federal Coordinator For Meteorological Services and Supporting Research|date=May 2010|accessdate=October 31, 2023|title=National Hurricane Operations Plan (FCM-P12-2010)|url=http://www.ofcm.gov/nhop/10/pdf/2010%20NHOP%20entire%20document.pdf|archivedate=July 8, 2011|archiveurl=https://web.archive.org/web/20110708201653/http://www.ofcm.gov/nhop/10/pdf/2010%20NHOP%20entire%20document.pdf}}|group=nb}} around 21:00 UTC.{{sfn|Richard et al.|2005|p=2}} NOAA aircraft reconnaissance recorded surface winds of {{convert|74|mph|km/h|abbr=on}} and Doppler weather radar velocities returned winds of {{convert|104|mph|km/h|abbr=on}} at a height of {{convert|3000|ft|m|abbr=on}}.{{cite report|first=Stacy R.|last=Stewart|publisher=National Hurricane Center|date=August 25, 2005|accessdate=December 27, 2021|title=Hurricane Katrina Discussion Number 9|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.009.shtml|type=Technical Discussion}} The upper-level ridge strengthened slightly as Katrina approached South Florida, causing the hurricane to turn west-southwest.{{sfn|Richard et al.|2005|p=2}} 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.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3913}}

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.{{sfn|Richard et al.|2005|p=2}}{{cite web|publisher=National Weather Service Forecast Office in Miami, Florida|year=2005|accessdate=December 20, 2021|title=Katrina|url=https://www.weather.gov/mfl/katrina}} 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).{{sfn|Richard et al.|2005|p=17}}{{sfn|Richard et al.|2005|p=2}} A weather station in Virginia Key measured sustained winds of {{convert|72|mph|km/h|abbr=on}}, and the NHC office—which went through the center of Katrina's eye—had sustained winds of {{convert|69|mph|km/h|abbr=on}} and a gust to {{convert|87|mph|km/h|abbr=on}}.{{sfn|Richard et al.|2005|p=5}} A minor storm surge occurred along the southeastern Florida coast.{{sfn|Richard et al.|2005|p=10}} 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.{{sfn|Richard et al.|2005|p=2}} 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.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3916}} This phenomenon is called the brown ocean effect. Meteorologists described Katrina as "still...an impressive cyclone."{{cite report|first=Richard D.|last=Knabb|publisher=National Hurricane Center|date=August 26, 2005|accessdate=December 27, 2021|title=Hurricane Katrina Discussion Number 11|url=https://www.nhc.noaa.gov/archive/2005/dis/al122005.discus.011.shtml|type=Technical Discussion}}

{{clear}}

File:Hurricane Katrina August 22, 2005, Gulf of Mexico dynamic height.jpg (altimetry-derived sea heights and water current flows on August 22 pictured).{{sfn|Chen et al.|2018|pp=293–294}}{{sfn|Jaimes|Shay|2009|p=4195}}|alt=A map of water heights in the Gulf of Mexico shaded from blue to yellow to orange (lowest to highest).]]

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.{{sfn|Richard et al.|2005|p=2}} The Dry Tortugas observed sustained hurricane-force winds, with a peak gust of {{convert|105|mph|km/h|abbr=on}},{{sfn|Richard et al.|2005|p=5}} 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.{{sfn|Richard et al.|2005|p=3}} Sea surface temperatures averaged {{convert|30|C|F|abbr=on|disp=flip}} across much of the Gulf, providing the hurricane with ample fuel. Furthermore, localized temperatures exceeding {{convert|31|C|F|abbr=on|disp=flip}} were present within two warm-core eddies that broke off the Gulf Loop Current.{{sfn|Chen et al.|2018|pp=293–294}}{{sfn|Jaimes|Shay|2009|p=4195}} 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.{{sfn|Kafatos et al.|2006|p=5}} Furthermore, Noel Davidson et al. (2008) hypothesized that an upstream Rossby wave influenced this intensification.{{cite conference|first1=Noel E.|last1=Davidson|first2=Chi Mai|last2=Nguyen|first3=Michael J.|last3=Reeder|date=April 30, 2008|accessdate=December 19, 2021|title=Downstream development during the rapid intensification of hurricanes Opal and Katrina: the distant trough-interaction problem|conference=28th Conference on Hurricanes and Tropical Meteorology|publisher=American Meteorological Society|number=9B.4|url=https://ams.confex.com/ams/28Hurricanes/techprogram/paper_138060.htm}} 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.{{sfn|Richard et al.|2005|p=3}} Of the five primary factors for rapid intensification described by Kaplan and DeMaria (2003), conditions were optimal for four of them.{{cite conference|first=Michael P.|last=Erb|date=April 6–8, 2008|accessdate=December 19, 2021|title=A Case Study of Hurricane Katrina: Rapid Intensification in the Gulf of Mexico|url=http://www.atms.unca.edu/chennon/research/documents/erb_ncur2006_preprint.pdf|conference=Proceedings of The National Conference on Undergraduate Research|publisher=University of North Carolina at Asheville|archive-date=December 19, 2021|archive-url=https://web.archive.org/web/20211219212153/http://www.atms.unca.edu/chennon/research/documents/erb_ncur2006_preprint.pdf|url-status=dead}}

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.{{sfn|Richard et al.|2005|p=3}} Significant lightning outbursts occurred in Katrina's eyewall throughout the rapid intensification phase.{{sfn|Fierro et al.|2011|p=1498}} 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.{{sfn|Richard et al.|2005|p=5}} 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.{{sfn|Richard et al.|2005|p=3}} Upweling of cold-core eddies occurred in the hurricane's wake, with sea surface temperatures falling by {{convert|5|–|6|C-change|disp=flip|abbr=on}} near the Florida Keys.{{sfn|Chen et al.|2018|p=297}}

During the pause in Katrina's intensification on August 27, inner-core convection weakened and the storm's overall structure became more asymmetric.{{sfn|Nguyen et al.|2011|p=836}} The ridge that previously kept the hurricane on a west-southwest trajectory shifted east, causing Katrina to turn due west and later west-northwest on August 28.{{sfn|Richard et al.|2005|p=3}} This took the hurricane over the Gulf of Mexico's OHC maximum—OHC values reached 123 kJ cm^–2 and sea temperatures of {{convert|26|C|F|abbr=on|disp=flip}} extended to a depth of {{convert|110|m|ft|abbr=on|disp=flip}}{{rp|340}}{{sfn|Jaimes| Shay|2009|p=4195}}—providing ample fuel for intensification.{{sfn|Rappaport et al.|2010|p=1387}} Following the consolidation of its new eye, Katrina underwent a second, more intense period of rapid strengthening while still continuing to grow in size. Within a 12-hour period, the storm's winds rose from 115 mph (185 km/h) to 165 mph (265 km/h) as it reached Category 5 hurricane status on the SSHS.{{sfn|Richard et al.|2005|p=3}} Its central pressure fell by 32 mbar (hPa; 0.94 inHg) during the same time.{{sfn|Richard et al.|2005|p=6}} By the latter half of August 28, Katrina's tropical storm-force and hurricane-force winds expanded to 230 mi (370 km) and 105 mi (165 km) from its center, respectively.{{sfn|Richard et al.|2005|p=3}} The eye displayed a pronounced stadium effect at this time, with the eyewall sloping outward with height.{{cite web|first=Chris|last=Dolce|publisher=The Weather Channel|date=August 26, 2015|accessdate=December 13, 2021|title=Inside the Eye of a Hurricane (Photos)|url=https://weather.com/storms/hurricane/news/eye-hurricane-photos-20130809}} As during the first rapid intensification period, lightning was observed in the eyewall. Throughout both lightning outbreaks, a total of 684 strikes were detected by the Los Alamos Sferic Array. Despite occurring over a longer duration, the electrical activity was less than what was detected in Hurricane Rita.{{sfn|Fierro et al.|2011|p=1499}}

Katrina reached its peak intensity with maximum sustained winds of 175 mph (280 km/h) and a central pressure of 902 mbar (hPa; 26.63 inHg) around 18:00 UTC. Aircraft reconnaissance measured flight-level winds of {{convert|191|mph|km/h|abbr=on}} at the storm's peak. Katrina's estimated intensity was primarily based on this observation, with a 90 percent rule of thumb reduction used to calibrate for surface-level equivalence. Concurrent dropsonde measurements showed instantaneous surface winds up to {{convert|165|mph|km/h|abbr=on}}; however, the maximum values were not co-located with the peak flight-level winds. Furthermore, stepped-frequency microwave radiometer data yielded maximum surface winds of {{convert|162|mph|km/h|abbr=on}}.{{sfn|Richard et al.|2005|p=6}} Katrina's pressure ranked as the fifth-lowest on record in the Atlantic basin and the second-lowest in the Gulf of Mexico.{{HURDAT}}{{sfn|Richard et al.|2005|p=6}}{{#tag:ref|Katrina's minimum pressure was only surpassed by Gilbert in 1988 (888 mbar (hPa; 26.22 inHg)), the 1935 Labor Day hurricane (892 mbar (hPa; 26.34 inHg)), Allen in 1980 (899 mbar (hPa; 26.55 inHg)), and Camille in 1969 (900 mbar (hPa; 26.58 inHg)).{{HURDAT}}{{sfn|Richard et al.|2005|p=6}} In the Gulf of Mexico

specifically, only Camille was stronger.{{HURDAT}} Katrina was soon surpassed the same year by Rita the following month (895 mbar (hPa; 26.43 inHg)){{cite report|first1=Knabb D.|last1=Richard|first3=Jamie R.|last3=Rhome|first2=Daniel P.|last2=Brown|publisher=National Hurricane Center|date=September 14, 2011|accessdate=December 9, 2021|title=Hurricane Rita|series=Tropical Cyclone Report|url=https://www.nhc.noaa.gov/data/tcr/AL182005_Rita.pdf|page=5}}—Rita became the strongest storm on record in the Gulf of Mexico—and Wilma in October (882 mbar (hPa; 26.04 inHg)).{{sfn|Richard et al.|2005|p=6}}|group="nb"}} At its peak, Katrina's radius of maximum wind of {{convert|56|km|mi|abbr=on|disp=flip}} was among the largest for a hurricane of its intensity.{{cite conference|conference=27th Conference on Hurricanes and Tropical Meteorology|date=April 26, 2006|accessdate=August 7, 2023|title=Eyewall evolution of Hurricane Katrina near landfall using NEXRAD reflectivity and radial velocity data|first1=Kimberly D.|last1=Campo|first2=Thomas M.|last2=Rickenbach|publisher=American Meteorological Society|url=https://ams.confex.com/ams/pdfpapers/108456.pdf}}

File:Hurricane Katrina Eye viewed from Hurricane Hunter.jpg on August 28.|alt=A photograph inside the eye of a hurricane depicting a bowl-like structure with clouds extending out and up from the center.]]

Late on August 28, another eyewall replacement cycle began as Katrina turned due north toward Louisiana. However, an increase in wind shear and entrainment of dry air interrupted this cycle. As the inner eyewall collapsed, the outer one failed to consolidate as convection diminished on the western side of the hurricane. Accordingly, the hurricane dramatically weakened during the overnight hours of August 28 into August 29. Furthermore, increased interaction with land—creating additional friction-based shear—could have contributed to this weakening.{{sfn|Richard et al.|2005|pp=3–4}} A study by Edward Rappaport et al. published in Weather and Forecasting in 2010 indicated that large hurricanes, especially intense ones, in the Gulf of Mexico often weaken significantly before landfall.{{sfn|Rappaport et al.|2010|p=1380}} A significant factor in this is the notable reduction of OHC over the northern Gulf compared to the central Gulf. As large hurricanes move into this area, upwelling becomes more pronounced and sea surface temperatures typically drop by {{convert|1|C-change|abbr=on|disp=flip}} underneath the hurricane.{{sfn|Rappaport et al.|2010|p=1389}} Katrina fell below Category 5 status before 06:00 UTC and continued weakening to a low-end Category 4 hurricane by 09:00 UTC.{{sfn|Richard et al.|2005|p=17}}{{sfn|Richard et al.|2005|p=7}} Given the hurricane's approximate 30–35 mi (45–55 km) radius of maximum wind, the NHC indicated sustained Category 4 winds may have briefly occurred along the extreme southeastern coast of Louisiana.{{sfn|Richard et al.|2005|p=7}} With core of Katrina closer to land, clear data was obtainable from NEXRAD installations. Wind velocities reached {{convert|162|mph|km/h|abbr=on}} at 10:15 UTC in the hurricane's northeastern quadrant and steadily decreased over the following hours.

At 11:10 UTC, the storm made landfall near Buras, Louisiana. Based on analysis of aircraft reconnaissance data, the NHC estimated Katrina to have moved ashore with maximum winds of 125 mph (205 km/h) and a central pressure of 920 mbar (hPa; 27.17 inHg). Based on central pressure, Katrina was the third-most intense hurricane to strike the continental United States at the time.{{sfn|Richard et al.|2005|p=7}}{{#tag:ref|Katrina's pressure was later eclipsed by Hurricane Michael in 2018 which made landfall near Panama City, Florida, with a central pressure of 919 mbar (hPa; 27.14 inHg).{{cite report|first1=Jack L.|last1=Beven|first2=Robbie|last2=Berg|first3=Andrew H.|last3=Hagen|publisher=National Hurricane Center|date=May 17, 2019|accessdate=December 9, 2021|title=Hurricane Michael (AL142018)|series=Tropical Cyclone Report|url=https://www.nhc.noaa.gov/data/tcr/AL142018_Michael.pdf|page=6}}|group=nb}} By landfall, Katrina's eyewall replacement cycle had completed and its core started reorganizing. According to a 2006 presentation by Campo and Rickenbach, Katrina's landfall intensity was of "great debate". NEXRAD base velocities reached {{convert|159|mph|km/h|abbr=on}} as the hurricane moved ashore. Analysis of dropwindsonde data yielded maximum surface winds of {{convert|119|mph|km/h|abbr=on}} while Stepped Frequency Microwave Radiometer data yielded {{convert|114|mph|km/h|abbr=on}}.

Katrina weakened further as it crossed the Breton Sound, eventually making its final landfall near along the Louisiana–Mississippi border at 14:45 UTC. By this time, Katrina's maximum winds fell to 120 mph (195 km/h) and its central pressure rose to 928 mbar (hPa; 27.40 inHg).{{sfn|Richard et al.|2005|p=17}} A University of Southern Mississippi buoy over the Mississippi Bight measured ten-minute sustained winds of {{convert|76.9|mph|km/h|abbr=on}} and a peak gust of {{convert|107.4|mph|km/h|abbr=on}} as the center of Katrina passed {{convert|56|mi|km|abbr=on}} to the west. The buoy was dragged with its anchor {{convert|13|km|mi|abbr=on|disp=flip}} southeast during this time. Subsequent analysis of data indicated wave amplitude and buoy tilt could result in a low-bias of NCDC buoy measurements of high-end wind events.{{sfn|Howden et al.|2008|pp=613–614}} Land-based measurements of Katrina's winds are incomplete in the landfall regions; anemometers failed either due to power outages or storm surge hours before the onset of the strongest winds. The highest officially verified gust was {{convert|124|mph|km/h|abbr=on}} in Grand Isle, Louisiana, while an unverified gust to {{convert|135|mph|km/h|abbr=on}} was observed in Poplarville, Mississippi. Located on the western side of Katrina, New Orleans experienced weaker winds. The NHC estimated the city likely experienced sustained Category 1– to Category 2–force winds. A station along the Lake Pontchartrain Causeway measured sustained winds of {{convert|78|mph|km/h|abbr=on}}. As winds increase with elevation, the high-rise buildings of the city may have experienced Category 2– to Category 3–force winds.{{sfn|Richard et al.|2005|p=8}}

{{see also|National Weather Service bulletin for Hurricane Katrina}}

On August 26, it became clear to forecasters at the NHC that Katrina was tracking significantly more west of the originally expected landfall in the Florida Panhandle. They shifted their forecast path 170 mi (275 km) west during the afternoon to the Mississippi/Alabama border and further to Louisiana and Mississippi that night. NHC director Max Mayfield called "[the one] we’ve been talking about all of these years,"{{cite web|type=Report|agency=Committee on Homeland Security and Governmental Affairs|publisher=Government of the United States|year=2006|accessdate=November 26, 2021|title=Hurricane Katrina: A National Still Unprepared|url=https://www.govinfo.gov/content/pkg/CRPT-109srpt322/pdf/CRPT-109srpt322.pdf|isbn=0-16-076749-0}} referencing the long-talked of worst-case scenario for New Orleans called "The Big One".{{cite news|publisher=CNN|date=August 29, 2005|accessdate=November 26, 2021|title=New Orleans braces for 'the big one'|url=http://edition.cnn.com/2005/WEATHER/08/28/katrina.neworleans/}} Mayfield personally called the governors of Louisiana and Mississippi emphasizing the catastrophic threat the storm posed, something he had only done once before in his 33-year career. By the morning of August 27, forecasters anticipated a direct hit on New Orleans. Following the westward shift, the NHC's forecasts were "exceptionally accurate and consistent". Landfall as a major hurricane was accurately pegged three days ahead of time. Track forecast errors were less than half of the 10-year average for 2005. Four- to five-day errors were slightly larger than the shorter-term overall; however, this was largely related to the west-southwest track shift across the Florida Panhandle rather than Katrina's path in the days leading up to landfall along the Gulf Coast.{{sfn|Richard et al.|2005|p=14}}

{{Gallery

|title=NHC graphical forecasts for Hurricane Katrina depicting the gradual westward shift in its expected path from August 23 to 26

|align=center

|File:Katrina-track-08-23-2005.jpg

|alt1=A map of a storm track surrounded by an error cone extending from the Bahamas northwest to Florida

|August 23: The NHC forecast the storm to move along a northwest and later west trajectory into the Florida Peninsula

|File:Hurricane Katrina forecast graphic 5.gif

|alt2=A map of a storm track surrounded by an error cone extending from the Bahamas west to Florida

|August 24: forecasts remained largely consistent

|File:Hurricane Katrina forecast graphic 10.gif

|alt3=A map of a storm track surrounded by an error cone extending from South Florida arcing north to the Florida Panhandle

|August 25: forecasts remained easterly, with an eventual landfall in the Florida Panhandle

|File:Hurricane Katrina forecast graphic 14.gif

|alt4=A map of a storm track surrounded by an error cone extending from the Florida Keys arcing north to Alabama

|Afternoon of August 26: forecasts shifted significantly west to the Mississippi/Alabama coast

|File:Hurricane Katrina forecast graphic 15.gif

|alt5=A map of a storm track surrounded by an error cone extending from the Florida Keys arcing north to Louisiana

|Evening of August 26: forecasts shifted farther west, with Katrina posing an imminent threat to Louisiana and Mississippi.

}}

File:FEMA - 17187 - Photograph by John Fleck taken on 10-04-2005 in Mississippi.jpg

{{further|2005 levee failures in Greater New Orleans}}

Hurricane Katrina produced a catastrophic storm surge across coastal Alabama, Louisiana, and Mississippi. The largest surge occurred in the right-front quadrant of the hurricane in Mississippi. Though the total destruction of buildings and infrastructure in coastal communities made post-storm assessments of surge heights difficult, a maximum high water mark of {{convert|27.8|ft|m|abbr=on}} was found in Pass Christian.{{sfn|Richard et al.|2005|p=9}} This was the largest storm surge observed in modern history in the United States.{{sfn|Needham and Keim|2014|p=2}} Farther east, surge heights reached {{convert|17|to|22|ft|m|abbr=on}} in eastern Mississippi and {{convert|10|to|15|ft|m|abbr=on}} in Alabama. Seawater penetrated as much as {{convert|6|mi|km|abbr=on}} inland on beaches and up to {{convert|12|mi|km|abbr=on}} inland along bays and rivers.{{sfn|Richard et al.|2005|p=9}}The hurricane's massive storm surge is attributed to its intensity the day prior and immense size, with its weakening prior to landfall having little effect. Northward swells propagated ahead of the hurricane and enhanced water levels.{{sfn|Richard et al.|2005|p=10}} The hurricane's perpendicular movement toward the coast across the shallow continental shelf further enhanced the surge.{{sfn|Camelo and Mayo|2021|p=1}} A National Data Buoy Center (NDBC) buoy anchored {{convert|74|mi|km|abbr=on}} south of Dauphin Island, Alabama, measured a peak significant wave height of {{convert|55|ft|m|abbr=on}}. This was the largest such measurement from a NDBC buoy on record.{{sfn|Richard et al.|2005|p=10}} Another buoy south of Horn Island measured a peak wave height of {{convert|10.73|m|ft|abbr=on}}.{{sfn|Bender III et al.|2010|p=1018}} The Mars oil platform—designed to withstand winds of {{convert|140|mph|km/h|abbr=on}} and waves up to {{convert|70|ft|m|abbr=on}}—sustained extensive damage as the eastern eyewall of Katrina passed over it. In a 2008 study by Wang and Oey, a wave height hindcast simulated maximum wave heights of {{convert|24|m|ft|abbr=on|disp=flip}} in the region of the platform.{{sfn|Wang and Oey|2008|pp=488, 494}} On the west side of the eye, northerly winds generated significant water rises along the south shores of Lake Pontchartrain. Surge heights reached {{convert|14|to|19|ft|m|abbr=on}} in New Orleans, St. Bernard and Plaquemines parishes. New Orleans' levee system was overwhelmed by the rising waters, with overtopping occurring in many locations. This overtopping subsequently undermined the backside of the levees and caused their failure. The ensuing floods ultimately inundated 80 percent of New Orleans, with flood waters persisting until October 11.{{sfn|Richard et al.|2005|p=9}}

Prior to Katrina, little emphasis was placed on the size of a hurricane in relation to the storm surge it would produce. Subsequent studies spurred by Katrina's devastation found significant correlation between storm size and surge. A 2008 study published in the Journal of Physical Oceanography found that surge values could differ as much as 30 percent of storms with similar intensities but different sizes. The paper asserted that the Saffir–Simpson scale was not a good indicator of coastal surge.{{sfn|Irish et al.|2008|p=2003}} As a direct result of Katrina and subsequent storms which produced large storm surges, the NHC revised the SSHS in 2010 to remove expected surge values and created a new product to convey forecast surge heights.{{sfn|Walker et al.|2018|p=1709}}{{sfn|Camelo and Mayo|2021|p=2}}{{cite news|first=Jack|last=Williams|newspaper=The Washington Post|date=September 7, 2012|accessdate=December 9, 2021|title=Why were hurricane categories established? Behind the Saffir-Simpson scale|url=https://www.washingtonpost.com/blogs/capital-weather-gang/post/why-were-hurricane-categories-established-behind-the-saffir-simpson-scale/2012/09/07/a05512aa-f8fd-11e1-8398-0327ab83ab91_blog.html}} A 2014 study published in Earth Interactions showed strong correlation between the radius of a system's storm- and hurricane-force winds 18 hours before landfall and storm surge heights.{{sfn|Needham and Keim|2014|p=12}}

File:Hurricane Katrina preliminary tornado reports.png

{{see also|Hurricane Katrina tornado outbreak}}

Throughout Katrina's progression across the United States, it produced a significant tornado outbreak with 57 tornadoes touching down across 8 states from August 26 to 31. The strongest of these were rated F2 on the Fujita scale: one on August 26 and five on August 29.{{cite web|publisher=National Centers for Environmental Information|year=2005|access-date=December 14, 2021|title=[United States Tornado Events for August 26–31, 2005]|url=http://www.ncdc.noaa.gov/stormevents/listevents.jsp?eventType=%28C%29+Tornado&beginDate_mm=08&beginDate_dd=26&beginDate_yyyy=2005&endDate_mm=08&endDate_dd=31&endDate_yyyy=2005&hailfilter=0.00&tornfilter=0&windfilter=000&sort=DT&submitbutton=Search&statefips=-999%2CALL|url-status=dead|archive-url=https://web.archive.org/web/20160304211205/http://www.ncdc.noaa.gov/stormevents/listevents.jsp?eventType=(C)+Tornado&beginDate_mm=08&beginDate_dd=26&beginDate_yyyy=2005&endDate_mm=08&endDate_dd=31&endDate_yyyy=2005&hailfilter=0.00&tornfilter=0&windfilter=000&sort=DT&submitbutton=Search&statefips=-999,ALL|archive-date=March 4, 2016}} Most of the tornadoes occurred in the hurricane's right-front quadrant, where strong low-level wind shear, high moisture content, and low convective available potential energy combined to produce locally favorable tornadic conditions.{{sfn|Lee et al.|2008|p=2}} The most prolific activity occurred on August 29 with 39 confirmed tornadoes; 18 of these were in Georgia, a daily record in the state for the month of August.{{cite web|type=Report|first1=J. Robby|last1=Westbrook |first2=James D.|last2=Westbrook |agency=National Weather Service Office in Peachtree City, Georgia|publisher=National Oceanic and Atmospheric Administration|date=August 29, 2005|access-date=December 14, 2021|title=Georgia's Record Tornado Outbreak|url=http://www.crh.noaa.gov/images/ffc/pdf/katrina.pdf}} A study published in 2008 documented 23 supercells in the six hours leading up to Katrina's landfall in Louisiana.{{sfn|Lee et al.|2008|p=2}} Localized winds within mesovortices may have reached Category 4 hurricane-equivalent intensity. Contrary to previous studies, the majority of these remained over water.{{sfn|Lee et al.|2008|p=1}}

Once onshore, Katrina rapidly weakened as it traversed Mississippi. Its winds subsided below hurricane-force by 00:00 UTC on August 30 as its center passed northwest of Meridian.{{sfn|Richard et al.|2005|p=4}} Interaction with an upstream trough caused Katrina to turn northeast across the Tennessee Valley and Ohio River Valley on August 30.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3906}}{{sfn|Richard et al.|2005|p=4}} The storm degraded to a tropical depression by 12:00 UTC.{{sfn|Richard et al.|2005|p=4}} Strong westerly shear produced by the trough caused Katrina's upper-level anticyclone to break east away from the low-level circulation on August 30. This led to the separation of the storm's low-level warm core which continued northeast and almost instantaneously transitioned into an extratropical cyclone that day as colder air wrapped in from the northwest.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3922}} The NHC assessed Katrina's dissipation as a distinct entity by 12:00 UTC on August 31 as it was absorbed into the aforementioned trough over central Ohio.{{sfn|Richard et al.|2005|p=17}} The remnants of Katrina continued across Pennsylvania and New York, eventually crossing the St. Lawrence River into southeastern Canada on September 1.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3906}} A stationary low over James Bay caused ex-Katrina to turn north and meander over Quebec for several days.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3906}}{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3922}} The storm deepened slightly during this time, with its central pressure falling by 10 mbar (hPa; 0.29 inHg), though its winds did not increase.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3922}} By September 3, the system acquired a more northeasterly to easterly heading, crossing over Newfoundland and Labrador and then emerging over the Labrador Sea on September 4.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3923}} The degrading system continued east and ultimately dissipated off the southern tip of Greenland on September 7.{{sfn|McTaggart-Cowan et al. (Part 1)|2007|p=3906}}

The anticyclone that separated from Katrina on August 30 in conjunction with the frontal boundary that the hurricane merged with had expansive impacts on weather across the Northern Hemisphere over the next two weeks.{{sfn|McTaggart-Cowan et al. (Part 2)|2007|p=3927}} The interaction of those systems created an upper-level pool of warm air that traveled east-northeast from the Southeastern United States on September 2, to the Azores on September 6, to northern Europe on September 9, and finally to the Tibetan Plateau on September 12 where it dissipated.{{sfn|McTaggart-Cowan et al. (Part 2)|2007|p=3934}} Northerly flow of tropical moisture and vorticity related to this system influenced the development of Hurricane Nate and the intensification of Hurricane Maria over the Central Atlantic.{{sfn|McTaggart-Cowan et al. (Part 2)|2007|pp=3935–3940}}

{{Gallery

|title=Satellite imagery of Hurricane Katrina as it progressed inland over North America

|align=center

|File:Katrina 2005-08-30 1645Z.jpg

|alt1=A satellite image of a tropical cyclone over the United States with cyclonically organized clouds

|August 30: Increasing shear displaced convection northeast of Katrina's center

|File:Katrina 2005-08-31 1552Z.png

|alt2=A satellite image of an extended area of clouds oriented southwest to northeast across the Great Lakes region of the United States and Canada

|August 31: Katrina's circulation became increasingly elongated as it became extratropical

|File:Katrina 2005-09-01 1637Z.jpg

|alt3=A satellite image of a large non-tropical cyclone over Eastern Canada with cyclonically organized clouds

|September 1: Katrina's extratropical remnant deepens over eastern Canada

|File:Katrina 2005-09-02 1543Z.jpg

|alt4=A satellite image of a large non-tropical cyclone over Eastern Canada with cyclonically organized clouds with a north to south oriented line of clouds well east of the storm

|September 2: The remnant system began weakening again as it turned east across Labrador

}}

{{Reflist|group=nb}}

{{commons|Hurricane Katrina}}

• List of Category 5 Atlantic hurricanes

{{reflist}}

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{{Katrinaseries}}

{{DEFAULTSORT:Meteorological History Of Hurricane Katrina}}

Category:Hurricane Katrina

Katrina

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Katrina meteorological history