 Hurricane (5) Katrina | |
HURRICANE KATRINA
23-30 AUGUST 2005
Katrina was an extraordinarily powerful and deadly hurricane that carved a wide swath of catastrophic damage and inflicted large loss of life. It was the costliest and one of the five deadliest hurricanes to ever strike the United States. Katrina first caused fatalities and damage in southern Florida as a Category 1 hurricane on the Saffir-Simpson Hurricane Scale. After reaching Category 5 intensity over the central Gulf of Mexico, Katrina weakened to Category 3 before making landfall on the northern Gulf coast. Even so, the damage and loss of life inflicted by this massive hurricane in Louisiana and Mississippi were staggering, with significant effects extending into the Florida panhandle, Georgia, and Alabama. Considering the scope of its impacts, Katrina was one of the most devastating natural disasters in United States history.
a. Synoptic History
The complex genesis of Katrina involved the interaction of a tropical wave, the middle tropospheric remnants of Tropical Depression Ten, and an upper tropospheric trough. This trough, located over the western Atlantic and the Bahamas, produced strong westerly shear across Tropical Depression Ten, causing it to degenerate on 14 August approximately 825 n mi east of Barbados. The low-level circulation gradually weakened while continuing westward, and it eventually dissipated on 21 August in the vicinity of Cuba. Meanwhile, a middle tropospheric circulation originating from Tropical Depression Ten lagged behind and passed north of the Leeward Islands on 18-19 August. A tropical wave moved through the Leeward Islands and merged with the middle tropospheric remnants of Tropical Depression Ten on 19 August, forming a large area of showers and thunderstorms north of Puerto Rico. This activity continued to move slowly northwestward, passing north of Hispaniola and then consolidating just east of the Turks and Caicos during the afternoon of 22 August. Dvorak satellite classifications from the Tropical Analysis and Forecast Branch (TAFB) of the Tropical Prediction Center (TPC) began at 1800 UTC that day. The upper tropospheric trough weakened as it moved westward toward Florida, and the shear relaxed enough to allow the system to develop into a tropical depression by 1800 UTC 23 August over the southeastern Bahamas about 175 n mi southeast of Nassau. The depression was designated Tropical Depression Twelve rather than “Ten” because a separate tropical wave appeared to be partially responsible for the cyclogenesis, and, more importantly, the low-level circulation of Tropical Depression Ten was clearly not involved.
The depression continued to become organized over the central Bahamas during the evening of 23 August. Deep convection increased overnight in the eastern semicircle of the cyclone and formed a well-defined band that began to wrap around the north side of the circulation center early on the morning of 24 August. Based on aircraft reconnaissance flight-level wind data, the cyclone became Katrina, the 11th tropical storm of the 2005 Atlantic hurricane season, at 1200 UTC 24 August when it was centered over the central Bahamas about 65 n mi east-southeast of Nassau. Initially the storm moved northwestward within a weakness in the lower tropospheric subtropical ridge. However, as the storm developed an inner core and evolved into a deeper cyclone on 24 August, it came under the influence of a strengthening middle to upper tropospheric ridge over the northern Gulf of Mexico and southern United States. This ridge turned Katrina westward on 25 August toward southern Florida. Katrina generated an intense burst of deep convection over the low-level center during the afternoon of 25 August while positioned over the northwestern Bahamas. Further strengthening ensued, and Katrina is estimated to have reached hurricane status near 2100 UTC 25 August, less than two hours before its center made landfall on the southeastern coast of Florida.
The strengthening ridge over the northern Gulf of Mexico and southern United States produced northeasterly middle- to upper-level tropospheric flow that forced Katrina to turn west-southwestward as it neared southern Florida. Katrina made its first landfall in the United States as a Category 1 hurricane on the Saffir- Simpson Hurricane Scale, with maximum sustained winds of 70 knots, near the border of Miami-Dade County and Broward County at approximately 2230 UTC 25 August. While not discernible in conventional satellite imagery, a well-defined eye became evident on the Miami National Weather Service (NWS) WSR-88D Doppler radar just prior to landfall on the southeastern Florida coast. In fact, the eye feature actually became better defined while Katrina moved inland, and it remained intact during its entire track across the peninsula. The convective pattern of Katrina as it crossed southern Florida was rather asymmetric due to northerly wind shear, which placed the strongest winds and heaviest rains south and east of the center in Miami-Dade County. Katrina continued west-southwestward overnight and spent only about six hours over land, mostly over the water-laden Everglades. Surface observations and velocity estimates from the Miami and Key West Doppler radars indicated that Katrina weakened over mainland Monroe County to a tropical storm with maximum sustained winds of 60 knots. The center of Tropical Storm Katrina then emerged into the southeastern Gulf of Mexico at approximately 0500 UTC on 26 August just north of Cape Sable.
Once back over water, Katrina quickly regained hurricane status at 0600 UTC with maximum sustained winds of 65 knots. Even though the center of Katrina continued westsouthwestward over the southeastern Gulf of Mexico and away from the southern Florida peninsula, a strong and well-defined rain band impacted large portions of the Florida Keys with tropical storm-force winds for much of the day on 26 August. Sustained hurricane-force winds were briefly measured at Dry Tortugas on the far western end of the island chain that afternoon.
Situated beneath a very large upper-level anticyclone that dominated the entire Gulf of Mexico by 26 August, resulting in very weak wind shear and efficient upper-level outflow, Katrina embarked upon two periods of rapid intensification (defined as a 30 kt or greater intensity increase in a 24-h period) between 26 and 28 August. The first period involved an increase in the maximum sustained winds from 65 kt to 95 kt in the 24-h period ending 0600 UTC 27 August. An eye became clearly evident in infrared satellite imagery early on 27 August, and Katrina became a Category 3 hurricane with 100 kt winds at 1200 UTC that morning about 365 n mi southeast of the mouth of the Mississippi River. During the remainder of the day, the inner eyewall deteriorated while a new, outer eyewall formed, and the intensity leveled off at 100 kt. Accompanying the intensification and the subsequent deterioration of the inner eyewall was a significant expansion of the wind field on 27 August. Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. The strong middle- to upper-tropospheric ridge that had kept Katrina on a west-southwestward track over the Florida peninsula and southeastern Gulf of Mexico began to shift eastward toward Florida, while a mid-latitude trough amplified over the north-central United States. This evolving pattern resulted in a general westward motion on 27 August and a turn toward the northwest on 28 August when Katrina moved around the western periphery of the retreating ridge. As Katrina churned westward on 27 August, it produced tropical storm force winds and heavy rainfall over portions of western Cuba. The new eyewall contracted into a sharply-defined ring by 0000 UTC 28 August, and a second, more rapid intensification then occurred. Katrina strengthened from a low-end Category 3 hurricane to a Category 5 in less than 12 h, reaching an intensity of 145 kt by 1200 UTC 28 August. Katrina attained its peak intensity of 150 kt at 1800 UTC 28 August about 170 n mi southeast of the mouth of the Mississippi River. The wind field continued to expand on 28 August, and by late that day tropical stormforce winds extended out to about 200 n mi from the center, and hurricane-force winds extended out to about 90 n mi from the center, making Katrina not only extremely intense but also exceptionally large.
The new eyewall that formed late on 27 August and contracted early on 28 August began to erode on its southern side very late on 28 August, while another outer ring of convection consolidated. These structural changes likely contributed to the rapid weakening that was observed prior to final landfall. Katrina turned northward, toward the northern Gulf coast, around the ridge over Florida early on 29 August. The hurricane then made landfall, at the upper end of Category 3 intensity with estimated maximum sustained winds of 110 kt, near Buras, Louisiana at 1110 UTC 29 August. Katrina continued northward and made its final landfall near the mouth of the Pearl River at the Louisiana/Mississippi border, still as a Category 3 hurricane with an estimated intensity of 105 kt. The rapid weakening of Katrina, from its peak intensity of 150 kt to 110 kt during the last 18 h or so leading up to the first Gulf landfall, appears to have been primarily due to internal structural changes, specifically the deterioration of the inner eyewall without the complete formation of a new outer eyewall. However, Katrina remained very large as it weakened, and the extent of tropical storm-force and hurricane-force winds was nearly the same at final landfall on 29 August as it had been late on 28 August. The weakening could have been aided by entrainment of dry air that was seen eroding the deep convection over the western semicircle while Katrina approached the coast. Gradually increasing wind shear, slightly lower ocean temperatures, and (following the first Gulf landfall) interaction with land each could also have played a role. Without extensive investigation, however, it is not possible to assess the relative roles played by these various factors. The weakening of major hurricanes as they approach the northern Gulf coast has occurred on several occasions in the past when one or more of these factors have been in place. Indeed, an unpublished study by the National Hurricane Center (NHC) reveals that, during the past 20 years, all 11 hurricanes having a central pressure less than 973 mb 12 h before landfall in the northern Gulf of Mexico weakened during these last 12 h.
Katrina weakened rapidly after moving inland over southern and central Mississippi, becoming a Category 1 hurricane by 1800 UTC 29 August. It weakened to a tropical storm about six hours later just northwest of Meridian, Mississippi. Katrina accelerated on 30 August, between the ridge over the southeastern United States and an eastward-moving trough over the Great Lakes. It turned northeastward over the Tennessee Valley and became a tropical depression at 1200 UTC 30 August. The depression continued northeastward and transformed into an extratropical low pressure system by 0000 UTC 31 August. The low was absorbed within a frontal zone later that day over the eastern Great Lakes.
b. Meteorological Statistics and Observations
Observations in Katrina include data from satellites, aircraft, airborne and ground-based radars, conventional land-based surface and upper-air observing sites, Coastal- Marine Automated Network (C MAN) stations, National Ocean Service (NOS) stations, ocean data buoys, and ships. Data from many Automated Surface Observing System (ASOS) sites, C-MAN stations, and buoys are incomplete due to power outages and other weather-induced failures prior to when peak winds and minimum pressures occurred.
Satellite observations include geostationary satellite-based Dvorak Technique intensity estimates from TAFB, the Satellite Analysis Branch (SAB), and the U. S. Air Force Weather Agency (AFWA). Microwave satellite data and imagery from National Oceanic and Atmospheric Administration (NOAA) near-polar-orbiting satellites, Defense Meteorological Satellite Program (DMSP) satellites, and National Aeronautics and Space Administration (NASA) satellites including the Tropical Rainfall Measuring Mission (TRMM), QuikSCAT, and Aqua, were also useful in tracking Katrina and assessing changes in its internal structure.
Aircraft reconnaissance missions were tasked on an almost continuous schedule from the genesis of Katrina until its final landfall. Observations from aircraft include flight-level and dropwindsonde data from 12 operational missions into Katrina, conducted by the 53rd Weather Reconnaissance Squadron of the U. S. Air Force Reserve Command, which produced 46 center fixes. Three missions were flown by the NOAA Aircraft Operations Center (AOC) Hurricane Hunter WP-3D aircraft, producing additional flight-level and dropwindsonde observations, 19 center fixes, real-time data from the Stepped Frequency Microwave Radiometer (SFMR), and airborne Doppler radar-derived wind analyses provided by NOAA’s Hurricane Research Division (HRD). Additionally, the NOAA G-IV jet conducted six synoptic surveillance missions during 24-29 August to collect dropwindsonde observations, primarily for enhancing the amount of data available to operational numerical models that provided guidance to NHC forecasters. An Air Force C-130J aircraft conducted one surveillance mission jointly with the G-IV on 25 August.
NWS WSR-88D Doppler radars across the southeastern United States and U. S. Department of Defense radars located in the Bahamas provided center fixes on Katrina. NWS WSR-88D velocity data were used to help estimate the intensity of Katrina when it was near or over land.
Katrina’s Florida landfall intensity of 70 kt near 2230 UTC 25 August is based on reduction to the surface of elevated velocities from the NWS Miami WSR-88D radar. The Miami radar, and 65 kt winds measured by the SFMR onboard a NOAA Hurricane Hunter aircraft, also indicated that Katrina had earlier become a 65 kt hurricane at about 2100 UTC. Due to Katrina’s asymmetric convective pattern as it passed over the Florida peninsula, the strongest winds occurred south and east of the center in Miami-Dade County. While the eye moved west-southwestward over northern Miami-Dade, it passed over the NWS Miami Forecast Office / National Hurricane Center facility near Sweetwater, Florida, where a pressure of 983 mb was measured at 0105 UTC 26 August. The eastern eyewall moved over the facility a few minutes later and sustained winds of 60 kt with a gust to 76 kt were measured near 0115 UTC. The strongest sustained wind measured by a land-based anemometer was 63 kt on Virginia Key. Doppler velocities from both the Miami and Key West WSR-88D radars suggest that maximum sustained surface winds were likely just less than hurricane strength while Katrina was centered over mainland Monroe County and while crossing the southwestern Florida coast. However, these data, combined with Dvorak satellite intensity estimates, indicate Katrina regained hurricane strength shortly after emerging over the Gulf of Mexico early on 26 August. Later that day, from about 1930 UTC to 2130 UTC, the Dry Tortugas C-MAN station (elevation 6 m) located about 60 n mi west of Key West, Florida reported sustained hurricane-force winds, as strong as 71 kt, with a gust to 91 kt. While sustained hurricane-force winds were not reported elsewhere in the Florida Keys, much of the island chain experienced sustained tropical storm-force winds with peak gusts between 60 and 70 kt while the center of Katrina passed to the north on 26 August.
Aircraft data indicate that Katrina continued to strengthen on 26 August, but concentric maxima in flight-level wind data and microwave imagery from several near-polar-orbiting satellites depict an eyewall replacement cycle that occurred on 27 August. This cycle temporarily prevented further strengthening, and aircraft data and Dvorak estimates indicate the intensity remained steady near 100 kt throughout that day. Katrina produced tropical storm-force winds in portions of western Cuba on 27 August, with gusts as strong as 54 kt and rainfall totals exceeding 8 inches in some locations. After the new eyewall consolidated and began to contract very early on 28 August, Katrina deepened that morning at a very rapid rate. Dropwindsonde observations from the Air Force and NOAA Hurricane Hunter aircraft indicate the central pressure fell 32 mb in 12 h, to 909 mb by 1200 UTC 28 August. The first wind observation to support Category 5 intensity was a peak 700 mb flight-level wind of 153 kt at about 1100 UTC 28 August, which corresponds to about 138 kt at the surface, using the 90% adjustment based on the mean eyewall wind profile derived from several past storms. The strongest flight-level wind measurement in Katrina was 166 kt near 1400 UTC that day, corresponding to about 150 kt at the surface. Dropwindsondes on 28 August provided surface wind estimates, derived from the mean wind over the lowest 150 m of the sounding, that were no greater than about 130-135 kt, but a few of these sondes directly measured 140-143 kt winds at 10 m. However, none of these sondes were released precisely from the point where flight-level winds were 166 kt, and it is also not likely that any of these sondes measured the maximum surface wind in the circulation. The SFMR, with a post-storm recalibration applied to compensate for a previous low bias at extremely high wind speeds, estimated surface winds as strong as 141 kt on the afternoon of 28 August, when maximum flight-level winds were about 160 kt. All available data from dropwindsondes and the SFMR indicate that, on average, the 90% adjustment of flight-level winds to the surface was valid until very late on 28 August.
The central pressure in Katrina fell to 902 mb near 1800 UTC 28 August. This pressure was (at the time) the fourth lowest on record in the Atlantic basin, behind 888 mb in Gilbert (1988), 892 mb in the Labor Day Hurricane of 1935, and 899 mb in Allen (1980). However, it has since quickly fallen to sixth lowest, following an observation of 897 mb in Hurricane Rita in September 2005 and the new record of 882 mb in Hurricane Wilma in October 2005. Based on the 902 mb pressure, and on the earlier 166 kt flight-level wind, the peak best track intensity of 150 kt is estimated to have occurred at 1800 UTC 28 August. [br]Postet at: February 06, 2006, 12:29:51 PM[hr]The extent, magnitude, and impacts of the damage caused by Katrina are staggering and are well beyond the scope of this report to fully describe. Thousands of homes and businesses throughout entire neighborhoods in the New Orleans metropolitan area were destroyed by flood. Strong winds also caused damage in the New Orleans area, including downtown where windows in some high rise buildings were blown out and the roof of the Louisiana Superdome was partially peeled away. The storm surge of Katrina struck the Mississippi coastline with such ferocity that entire coastal communities were obliterated, some left with little more than the foundations upon which homes, businesses, government facilities, and other historical buildings once stood. Despite being more distant from the eye of Katrina, the storm surge over Dauphin Island, Alabama destroyed or damaged dozens of beachfront homes and cut a new canal through the island’s western end. Many of the most severely impacted areas along the northern Gulf coast could take years to completely rebuild. Katrina’s heavy rains in southern Florida flooded some neighborhoods, primarily in Miami-Dade County. Many other structures from Florida and Georgia westward to Louisiana that avoided surge or fresh water floods, including some areas well inland, were damaged by strong winds and tornadoes. Considerable damage to some homes and agricultural facilities was caused by several tornadoes in Georgia. Strong winds caused significant tree damage throughout much of Mississippi and Alabama. Combining all of the areas it impacted, Katrina left about three million people without electricity, some for several weeks.
The economic and environmental ramifications of Katrina have been widespread and could in some respects be long-lasting, due to impacts on large population and tourism centers, the oil and gas industry, and transportation. The hurricane severely impacted or destroyed workplaces in New Orleans and other heavily populated areas of the northern Gulf coast, resulting in thousands of lost jobs and millions of dollars in lost tax revenues for the impacted communities and states. Along the Mississippi coast, several large casinos on floating barges were damaged or destroyed when the surge pushed them onshore. Large numbers of evacuees have not returned home, producing a shortage of workers for those businesses that have reopened. Major beach erosion occurred along the tourism-dependent coasts of Mississippi and Alabama. A significant percentage of United States oil refining capacity was disrupted after the storm due to flooded refineries, crippled pipelines, and several oil rigs and platforms damaged, adrift or capsized. An oil rig under construction along the Mobile River in Alabama was dislodged, floated 1.5 miles northward, and struck the Cochrane Bridge just north of downtown Mobile. An offshore oil rig washed up near the beach of Dauphin Island, Alabama. Several million gallons of oil were spilled from damaged facilities scattered throughout southeastern Louisiana. While several facilities have since resumed operations, as of this writing oil and natural gas production and refining capacity in the northern Gulf of Mexico region remains less than that prior to Katrina. Key transportation arteries were disrupted or cut off by the hurricane. Traffic along the Mississippi River was below normal capacity for at least two weeks following the storm. Major highways into and through New Orleans were blocked by floods. Major bridges along the northern Gulf coast were destroyed, including several in Mississippi and the Interstate 10 Twin Span Bridge connecting New Orleans and Slidell, Louisiana.
Estimates of the insured property losses caused by Katrina vary considerably and range between about $20 billion and $60 billion. The American Insurance Services Group (AISG) estimates that Katrina is responsible for $38.1 billion of insured losses in the United States. A preliminary estimate of the total damage cost of Katrina is assumed to be roughly twice the insured losses (using the AISG estimate), or about $75 billion. This figure would make Katrina far and away the costliest hurricane in United States history. Even after adjusting for inflation, the estimated total damage cost of Katrina is roughly double that of Hurricane Andrew (1992). Normalizing for inflation and for increases in population and wealth, only the 1926 hurricane that struck southern Florida surpasses Katrina in terms of damage cost. However, this would not be the case if the values on the higher end of the range of Katrina estimates are later found to be the most accurate. The Insurance Information Institute reports that, mostly due to Katrina but combined with significant impacts from the other hurricanes striking the United States this year, 2005 was by a large margin the costliest year ever for insured catastrophe losses in this country.
Data provided by the Federal Emergency Management Agency (FEMA) indicate that over 1.2 million people along the northern Gulf coast from southeastern Louisiana to Alabama were under some type of evacuation order, but it is not clear how many people actually evacuated. Media reports indicate that many displaced residents have moved either temporarily or permanently to other areas in the United States. A large number of these people might never return to live in their pre-Katrina homes or cities. Thousands of people are still living in hotels and temporary shelters as of this writing. Some people remain separated from other family members and/or are unable to determine if their family member(s) survived the storm.
d. Forecast and Warning Critique
The possibility of development of the system that eventually became Katrina was conveyed in the Tropical Weather Outlook (TWO) issued by the NHC, beginning about 36 hours before it became a depression on 23 August. Only gradual development was anticipated in the TWO until the incipient system began to consolidate early in the morning of 23 August, about 12 hours before the first advisory was issued. The possibility that a depression could form later in the day was conveyed in the 11:30 am EDT issuance of the TWO on 23 August.
Average official forecast track errors in nautical miles (n mi) (with number of cases in parentheses) for Katrina were 24 (27), 42 (25), 64 (23), 96 (21), 174 (17), 213 (13), and 244 (9) for the 12, 24, 36, 48, 72, 96, and 120 h forecasts respectively. These forecast errors were considerably less than the average official Atlantic track errors for the 10-year period 1995-2004 of 42, 75, 107, 138, 202, 236, 310 for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. Note that average errors at 96 and 120 h reflect only the period 2001-2004 when those forecasts had been made either experimentally (2001-02) or operationally (2003-04). The 12-48 h official forecasts during Katrina represented significant improvements of 43%, 44%, 40%, and 31%, respectively, over the corresponding 10-year averages. The relatively small errors at 12-48 hours greatly helped in the issuance of generally accurate and timely coastal watches and warnings. The 72, 96, and 120 h official track forecast errors were still less than the long-term averages, but only by 14%, 10%, and 25%, respectively. The errors at these longer lead times can be partially attributed to the difficult forecast scenario associated with Katrina’s west-southwestward motion across the southern Florida peninsula and eastern Gulf of Mexico.
Even though the official track forecasts were generally much better than the previous 10- year averages, the official forecasts were bettered on average at various lead times by several of the numerical models. Specifically, the interpolated United Kingdom Met. Office global model (UKMI), the Florida State University Superensemble (FSSE), and the CONU and GUNA consensus models on average provided forecasts that were more accurate than the official forecast out to 72 h. The interpolated versions of the GFDL and NOGAPS models, GFDI and NGPI, on average were more accurate for Katrina than the official forecast at 72 and 96 h; GFDI was also more accurate than the official forecast at 120 h. Contributing to the lower GFDI errors was the fact that, before Katrina crossed Florida, the GFDI forecast a more southerly track across the Florida peninsula and a track farther to the west over the Gulf of Mexico than the official forecast.
The official track forecasts for Katrina issued within about two and a half days of landfall of the center in Louisiana were exceptionally accurate and consistent. Every official forecast that was issued beginning at 2100 UTC 26 August showed a track crossing the coast of Mississippi and/or southeastern Louisiana. The NHC does not explicitly issue forecasts for the precise location or timing of landfall. However, it is instructive to examine the forecasts for Katrina verifying at 1200 UTC 29 August, less than an hour following the initial Louisiana landfall. The official track forecasts issued 12, 24, 36, and 48 hours prior to 1200 UTC 29 August were in error by only 19, 24, 32, and 56 n mi, respectively. These errors are less than half the magnitude of the corresponding 10-year averages among all Atlantic basin forecasts. Importantly, they were all issued with a hurricane watch or warning in effect for the northern Gulf coast. Much of the credit for these very small errors must be given to the guidance provided by several dynamical models during this time frame.
Average official intensity forecast errors during Katrina were 10, 17, 22, 28, 47, 43, and 36 kt for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively. These errors were considerably larger than the corresponding Atlantic 10-year (1995-2004) averages of 6, 10, 12, 15, 18, 20, and 22 kt. Despite the larger than average intensity errors, the official forecasts did provide some useful indications, especially with respect to the issued watches and warnings, of what the intensity could be at the initial United States landfall in Florida. Additionally, every official forecast within about three days of landfall in Louisiana correctly anticipated that Katrina would be a major hurricane (at least Category 3) at landfall on the northern Gulf coast. The average official intensity forecast errors at all lead times were less than those for the SHIPS model (with inland decay component included), the GFNI (interpolated GFDN), and the FSSE. However, the GFDI (interpolated GFDL) intensity forecasts were on average more accurate than the official intensity forecasts, except at 12 and 120 h. Katrina was an unusually intense hurricane and underwent two rapid intensification periods, including the very rapid strengthening from Category 3 to Category 5 on the morning of 28 August. Obviously, accurately forecasting the timing and magnitude of such events remains an operational challenge, in part because the available intensity guidance models generally have little or no skill in forecasting rapid intensity changes. However, the GFDI was quite accurate in forecasting, about two days in advance, that Katrina would reach the Category 2/3 intensity threshold early on 27 August. In fact, it was in error by only 2 kt with its 24-h forecast of Katrina’s strengthening from 65 kt to 95 kt by 0600 UTC 27 August. The GFDI model also consistently forecast Katrina to reach Category 4 intensity beyond that time and to remain a major hurricane until landfall on the northern Gulf coast. The GFDI fell short of forecasting at any lead time the strengthening of the winds from Category 3 to Category 5 intensity on 28 August. However, the model did forecast the central pressure to fall to about 920 mb beginning about two days before that occurred on 28 August, and it even forecast the pressure to fall below 910 mb about 24 hours in advance of when the 902 mb minimum pressure was observed.
A tropical storm warning was issued for the central and northwestern Bahamas at 2100 UTC 23 August, approximately 15 hours before Katrina strengthened into a tropical storm over the Bahamas. A tropical storm watch was issued on 24 August at 0300 UTC, approximately 43.5 hours prior to the landfall of the center of Katrina in Florida, for portions of the Florida Keys and the southern peninsula of Florida from Seven Mile Bridge northward to Vero Beach. A hurricane watch was later issued from Florida City to Vero Beach on 24 August at 1500 UTC, approximately 31.5 hours ahead of landfall of the center, and this was changed to a hurricane warning on August 25 at 0300 UTC, approximately 19.5 hours prior to landfall. The lead times on the hurricane watch and warning for southern Florida were less than the usual targets of 36 and 24 hours, respectively, since Katrina’s forward motion toward the peninsula later on 25 August was faster than forecast. However, the hurricane watch and warning were issued while Katrina was still a tropical storm. Despite the uncertainties in intensity forecasting, strengthening to a hurricane before the Florida landfall was correctly forecast from the time the hurricane watch was issued. Strengthening to near the threshold of hurricane status before landfall was also indicated in every advisory on Katrina issued prior to that time.
A tropical storm watch was issued for the middle and upper Florida Keys about two days before tropical storm conditions reached those areas. However, subsequent tropical storm watches and warnings for the Florida Keys and the southwestern coast of the Florida peninsula were issued with less than the desired lead times, due to the actual motion of Katrina across the peninsula being more rapid and more toward the west-southwest than forecast. A tropical storm warning was issued at 2100 UTC 25 August for the Florida Keys from Key West northeastward, and along the southwestern coast of Florida from Longboat Key southward, about nine hours before tropical storm-force winds began in portions of that warning area. Additionally, while a tropical storm warning was issued for Dry Tortugas at 0500 UTC 26 August, a hurricane warning was never issued for that island. Sustained hurricane-force winds were reported there for a brief period near 2100 UTC 26 August. However, sustained hurricane-force winds were not reported anywhere else in the Florida Keys.
A hurricane watch was initially issued for the Louisiana coast from Morgan City to the mouth of the Pearl River at the Mississippi border, on 27 August at 1500 UTC, approximately 44 hours in advance of the initial Louisiana landfall of the center of Katrina. This watch was extended eastward at 2100 UTC that day to include the entire coastlines of Mississippi and Alabama, more than 41 hours prior the landfall of the center near the Louisiana/Mississippi border. A hurricane warning was later issued from Morgan City to the Florida/Alabama border at 0300 UTC 28 August, approximately 32 hours in advance of the initial Louisiana landfall of the center and more than 35 hours prior to the final landfall of the center. These long lead times were necessary to account for the large extent of hurricane-force winds from the center, and the need to complete preparations before the even earlier arrival on the coast of tropical storm-force winds. The timing of the watches and warnings was especially prudent given that Katrina reached the coast slightly faster than forecast at the time they were issued. To allow for sufficient lead time while accounting for forecast uncertainty, these watch and warning areas were fairly large. However, since hurricane-force winds extended at least 75 n mi to the east from the center of Katrina on the morning of final landfall, if the watch and warning areas had been any smaller on the eastern end, portions of the coastline would not have been properly warned. Sustained hurricane-force winds were reported as far east as Dauphin Island, Alabama, leaving at most about 30 n mi to spare within the eastern end of the hurricane warning area.
e. Best Track for Hurricane Katrina
23 / 1800 23.1 075.1 1008 030 tropical depression
24 / 0000 23.4 075.7 1007 030 "
24 / 0600 23.8 076.2 1007 030 "
24 / 1200 24.5 076.5 1006 035 tropical storm
24 / 1800 25.4 076.9 1003 040 "
25 / 0000 26.0 077.7 1000 045 "
25 / 0600 26.1 078.4 0997 050 "
25 / 1200 26.2 079.0 0994 055 "
25 / 1800 26.2 079.6 0988 060 "
26 / 0000 25.9 080.3 0983 070 hurricane
26 / 0600 25.4 081.3 0987 065 "
26 / 1200 25.1 082.0 0979 075 "
26 / 1800 24.9 082.6 0968 085 "
27 / 0000 24.6 083.3 0959 090 "
27 / 0600 24.4 084.0 0950 095 "
27 / 1200 24.4 084.7 0942 100 "
27 / 1800 24.5 085.3 0948 100 "
28 / 0000 24.8 085.9 0941 100 "
28 / 0600 25.2 086.7 0930 125 "
28 / 1200 25.7 087.7 0909 145 "
28 / 1800 26.3 088.6 0902 150 "
29 / 0000 27.2 089.2 0905 140 "
29 / 0600 28.2 089.6 0913 125 "
29 / 1200 29.5 089.6 0923 110 "
29 / 1800 31.1 089.6 0948 080 "
30 / 0000 32.6 089.1 0961 050 tropical storm
30 / 0600 34.1 088.6 0978 040 "
30 / 1200 35.6 088.0 0985 030 tropical depression
30 / 1800 37.0 087.0 0990 030 "
31 / 0000 38.6 085.3 0994 030 extratropical
31 / 0600 40.1 082.9 0996 025 "
31 / 1200 00.0 000.0 0000 000 merged with front
Maximum Wind and Minimum Pressure
28 / 1800 26.3 088.6 0902 150
Landfall(s)
25 / 2230 26.0 086.1 0984 070 FL landfall at Broward/Miami-Dade County line
29 / 1110 29.3 089.6 0920 110 Landfall near Buras, LA
29 / 1445 30.2 089.6 0928 105 Landfall near LA/MS border
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Linear Mode
