Please analyze/explain why/how Katrina weakened

[Anonymous]

Hi
the coastline itself (i.e, getting shallower and shallower) and shape of the coastline (i.e , bays etc ) accentrated the storm surge, similar to what happened in Thailand with the Tsunami...
and so I dont think its a good way to compare or measure the strength of Katrina...i.e on a different coastline the storm surge would not have been as great...or in the open sea it would not have been as high...
Katrina was in the worst track for creating the highest storm surge, becuase of the wind angles and shape of the coast line, etc...
thats my 2 cent worth and what I have come away with, but I am no expert either!

I see the that is now a news item here:
http://wlox.com/Global/story.asp?S=3782622...22&nav=6DJHduXE
that it was the storms postion and orientation and the shape of the coastline etc that caused such a high storm surge...

 

[MClarkson]

I think you chalk up some of it to land interaction. When you start losing convection(in the western side where you have dry offshore flow) you decrease the ammount of air dirverging up and out of the storm, increasing surface pressure.

Also it appears a double eyewall had formed... this is a feature usually associated with an eyewall replacement cycle. I am not certain how far into the eyewall cycle she actually got before landfall... as the inner eyewall maintained its diameter rather well, and recon only had periodic reports of being open to the SW.

Weakening is of course a relative term... she still came in at 918mb.

 

[Dave Kaplow]

After being off this board for so long, I want to provide my $0.02.

Katrina weakened slightly before landfall, but that was normal. I want to clarify that IMHO it WAS NOT an ERC. It was a combination of a shortwave trough that induced some shear right before landfall and dry air entrainment, something that happens all the time when a Gulf Coast storm heads for landfall. The surge from Katrina was worse than Camille was because that Camille wasn't as large, and frankly, I too think that Camille was weakening at landfall, and perhaps a later analysis can even prove that Camille was NOT a Cat 5 at landfall.

Some other great examples of this phenomenon: Dennis, Ivan, Opal, and Lili. If it weren't for Katrina being a Cat 5 and its large envelope and its low pressure, then not even a Cat 4 would've made landfall.

The NHC tends to err on the side of caution though and forecast less weakening then what usually occurs.

An intriguing post. I tend to agree with most of what you say, except for the bit about no ERC. On the other hand, I'm by no means a tropical storm expert and my belief that an ERC was in progress was based mainly on the fact that many people whom I do consider experts were saying that it was so. AFAIK you are the first person to challenge the ERC theory. It will be interesting to see what the final judgment is, when the post-mortem analysis of this event is complete and all of the many studies it will generate are completed. What I saw was an eye that was obviously perturbed and breaking down and I just assumed ERC. Can you tell us more about why you believe there was no ERC at all?

Your comments about pre-landfall weakening being a well known process are also interesting. If this is truly the perceived wisdom of the professional meteorologists at NHC and elsewhere, then why did they keep using language like "there is nothing I know of that might weaken the storm prior to landfall", etc. (my words, but many of the discussions and forecasts I read used almost identical language). I think if this is a kind of open secret in the meteorological community, that many gulf hurricanes tend to weaken before landfall, it needs to be spoken of in forecasts and carefully explained to the public. If a forecaster truly believes in his heart that a storm is likely to weaken, he should say so in his forecasts, even if it's only a quick acknowledgment that historically this has been known to happen. It's all well and good to err on the side of caution, but a forecaster owes his audience and/or clients the truth.