Strongly disagree.
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"First of all, who downgrades a storm that killed 1,000+ peope to a Category Three. Look at the damage, what was done to New Orleans. A Category Three couldn't have done this. And the levees were supposed to withstand Category Three strength storms, but they broke, and they are blaming bad structuring for the breach but it's obvious that Katrina was stronger than a three."
The Cat. 5 version of Katrina was the one which created most of that surge. The fast translation speed coupled with the winds provided an ideal growth scenario for the waves and surge. It takes a lot longer for seas to subside than for winds to subside, and thus tons of wave energy hit the coastline. The levees were in poor condition before the storm and were breeched once the water became significant in the vicinity of the city. That, IMO, is not an argument that Katrina was stronger than a category 3 at landfall.
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Another thing, Katrina was still a Category 5 by Midnight and even though rapid weakening can be the cause, they could have gotten a reading from another part of the storm. Also Katrina had the 3rd lowest pressure of a storm at US landfall. The two that are lower are the Labor Day Hurricane and Camille who had less damage than Katrina. The top 3 before Katrina were all Cat.5 hurricanes, which would seem strange for a Cat. 3 to take that record.
1. Rapid weakening was the cause. They measured the winds all over the storm as it was approaching the coast with multiple airplanes taking both dropsonde and SFMR instruments, allowing for complete scanning of the storm. In fact, the analyses from the post-storm wind products suggest that the winds were even weaker than the NHC analyzed as it made landfall! (e.g. ftp://ftp.aoml.noaa.gov/hrd/pub/hwind/2005/PostKatrina/0829/0600/col08deg.png -- 108kt max surface wind, middle of category 3 intensity)
2. Pressure and maximum wind are not correlated directly. More important is how the pressure field is spread out over a short distance within the storm as well as how the pressure of the storm compares to that in the outer environment. Katrina had a very broad pressure and wind field distribution and thus supported a low pressure but relatively low wind speeds.
3. The Labor Day Hurricane and Camille were storms from different eras. If either hit in 2005, as did Katrina, it's a different story. Katrina was unique in that it leveled the most vulnerable city in the SE US with tons of water damage; the Labor Day hurricane was known merely for its intensity while Camille hit in a time period when the coastline was not nearly as developed as today. The storms are not directly comparable.
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Also, the pressure. 918 mb is a pressure that is associated with Category 4 and Category 5 hurricanes not Category Three storms. The lowest pressure in a Category Three was 939 mb. Opal's 135 mph / 916 mb has an excuse to keep it a four at the time, since Opal's pressures were lower than the matching wind speed, while Katrina's wind speeds caught up every time.
See my comments above (point 2) regarding the pressure-wind relationship. Katrina's wind speeds did not "catch up" every time to its pressure, either; while pressure changes generally do precede wind field changes, wind field changes can occur independent of numeric pressure changes -- such as by a broadening of the storm's circulation or entering into a different background environment. There's no indication in the data that Katrina's winds matched the low pressure it continued to display until landfall.
Values like damage and death toll are important values, but they are only partially correlated with storm intensity. The number one killer from tropical systems is water -- particularly inland flooding from torrential rains. Damage totals can be heavily skewed based upon where a storm made landfall -- a category 4 storm into S. Texas (like Bret a few years ago) will likely cause a lot less damage than a weak hurricane into S. Florida (like Katrina itself) solely because one area is relatively uninhabited while the other is heavily populated.
In summary, the data only supports Katrina as a category 3 hurricane at landfall and it is quite possible that the NHC's post-storm and post-season analyses are too high given the available data. The water which caused the decimation of much of the coastline started to get going while Katrina was a few hundred miles west of Key West; it did not just suddenly appear when the storm made landfall.
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Re: Who thinks Katrina was a Cat. 5 at landfall.
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