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Katrina officially downgraded to a Category 3

By: Dr. Jeff Masters, 4:08 PM GMT on December 21, 2005

Today is winter solstice--the darkest day of the year--and an appropriate time to revisit America's other darkest day of the year, August 29. The National Hurricane Center (NHC) issued its official Tropical Cyclone Report for Katrina on Tuesday. Katrina officially made landfall as a Category 3 hurricane, not a Category 4. Ground-based and aircraft measurements only support 110 knot winds (127 mph) at Katrina's first landfall near Buras, Louisiana. Katrina weakened only slightly before her second landfall, and was still a Category 3 hurricane with 105 knot (121 mph) winds on the Mississippi coast. The NHC report also stated that the highest sustained winds over metropolitan New Orleans were only of Category 1 or 2 strength, although buildings over 25 stories high may have seen winds a full category higher.

The reason Katrina was originally classified as a Category 4 at landfall was because winds measured by the Hurricane Hunters at flight level (10,000 feet) were 150-155 mph. The normal rule of thumb used to estimate surface winds is a 10% redution from the winds at 10,000 feet. This rule of thumb was applied for the official NHC advisories issued at the time of Katrina's landfall, and made Katrina a Category 4 hurricane with 135-140 mph surface winds. However, detailed analysis of the wind structure of Katrina in data gathered by Doppler radar and dropsondes showed that at landfall, Katrina had its highest winds in an unusually strong band of winds between 2 and 4 km (the flight level of the Hurricane Hunters was about 3 km). Normally, the highest winds in a hurricane are found much lower, near .5 - 2 km. Surface winds measured by dropsondes, surface towers, and the SFMR microwave radiometer on the aircraft all agreed that the surface winds at landfall were no higher than 100 knots (115 mph). NHC adjusted these upwards by 10% to account for the fact that the strongest winds were likely not sampled. The 10% adjustment left Katrina just 5 mph shy of Category 4 status--but still a very potent and deadly major Category 3 hurricane.

The 10% reduction "rule of thumb" was not valid for Katrina at landfall, probably because the storm's convection was weakening at that time. Because momentum transport from aloft to the surface was impaired by the weakening convention, Katrina was less able to carry the strong winds that were aloft down to the surface. Thus, winds at the surface were about 80% of the winds measured at 10,000 feet. Still, NHC does mention that given the uncertainties and large wind field of Katrina, the very tip of the Mississippi Delta near Buras may have received Category 4 winds for a few minutes, and it is possible Katrina really was a Cat 4 at landfall.

Why Katrina weakened at landfall
At peak intensity, Katrina was a Category 5 hurricane with 150 kt (174 mph) winds, but in the 18 hours before landfall weakened to 110 kt (121 mph). This weakening occured as a result of entrainment of dry air into the storm, slightly cooler sea surface temperatures near the coast, and interaction with the land. NHC notes that the relative importance of these three factors cannot be determined without a lot more study, but all 11 hurricanes with pressures less than 973 mb that have hit the Gulf coast the past 20 years have weakened in the 12 hours prior to landfall. Thus, Katrina's weakening should come as no surprise. Note, however, that Hurricane Camille of 1969 did not weaken when it pounded Mississippi as a Category 5 hurricane; perhaps it's small size protected it from substantial land interaction and entrainment of dry air.

Storm surge
To me, the biggest disappointment in the report came in the treatment of Katrina's storm surge. No storm surge data was presented for New Orleans. No mention was made that Katrina, despite its Category 3 strength at landfall, pushed a Category 5 level storm surge to the coast. The report noted that official storm surge measurements were unavailable, due to failure of most of the tide gauges. However, one unofficial storm surge height of 27 feet at the Hancock County Emergency Management Office in Mississippi was mentioned, which would make Katrina's storm surge the highest on record for an Atlantic hurricane. The previous record was Hurricane Camille's 24.7 feet. Any surge above 18 feet is considered a Category 5 level storm surge. I've seen unofficial estimates that the storm surge affecting the eastern side of New Orleans was 18-25 feet high, which is clearly a Category 5 storm surge. Not surprisingly, the levees protecting the east side of the city were overwhelmed and failed in multiple locations. However, observational data and computer modeling indicate that storm surge entering the canals from Lake Pontchartrain reached 9 to 11 feet in the 17th Street Canal and 11 to 12 feet in the London Avenue Canal. The flood walls were 13.5 feet high or higher along much of the two canals and were designed to withstand water rising to 11.5 feet. A Category 3 storm surge is 9-12 feet, so these flood walls failed in a Category 3 level storm surge, even though they were supposedly designed to withstand that type of storm surge.

Katrina officially made landfall at Buras, LA, with a central pressure of 920 mb. This is the third lowest pressure on record for a U.S. landfalling hurricane, surpassed only by the two Category 5 hurricane to hit the U.S.--the Florida Keys Labor Day Storm of 1935 (892 mb) and Hurricane Camille of 1969 (909 mb). Katrina had the lowest pressure ever measured for a Category 3 hurricane; the previous record was 930 mb for Hurricane Floyd of 1999. Katrina's unusually low winds were primarily due to the fact that Katrina was a huge storm--the change of pressure from outside the storm to inside the storm happened over a large distance. It's the pressure gradient--the change of pressure with distance--that drives winds, not the pressure itself.

Death Toll
The official death toll so far is 1336, with 1090 of those victims in Louisiana and 228 in Mississippi. This makes Katrina at least the fifth deadliest U.S. hurricane of all time. The death toll could go much higher, making Katrina the third deadliest. Over 4,000 people are still listed as missing. Most of these missing people are probably alive and well, according to Kym Pasqualini, CEO of National Center for Missing Adults. However, she indicates that 1,300 of the missing from the most heavily damaged areas of New Orleans are a matter of great concern, and many of these people may have died in the storm.

The report quotes a preliminary figure of $75 billion in damage for Katrina, a number used by the American Insurance Services Group (AISG). This would make Katrina, by a least a factor of two, the costliest hurricane ever. A recent estimate by the world's largest re-insurance company, the Swiss Munich Re Foundation, put Katrina's total damage closer to $125 billion.

Forecast accuracy
NHC gives themselves high marks for forecast accuracy for the 2 1/2 days prior to Katrina's landfall. Indeed, their landfall location forecasts had errors more than a factor of two better than average. These exceptionally accurate forecasts likely saved hundreds of lives. On the other hand, NHC intensity forecasts for Katrina were up to a factor of two worse than average, and perhaps more lives could have been saved had these intensity forecasts been better.

Jeff Masters
katrina surge
katrina surge
1st. Baptist Church Gulfport, MS
Katrina Damage at the 17th St Levee Break
Katrina Damage at the 17th St Levee Break
Hurricane Katrina Search and Rescue
Hurricane Katrina Search and Rescue
I work for Louisiana Wildlife and Fisheries. We are in search and rescue mode. Storm damaged and flooded house.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.