Please analyze/explain why/how Katrina weakened

Lately we've seen an awful lot of storms that have weakened significantly immediately before landfall, much more than chance alone can account for, or so it seems to me. I think there may be something else at work here, perhaps several somethings all contributing together, above and beyond the usual ERC fluctuations one expects to find in a major huricane at sea. We are going to have to understand this pre-landfall burst of weakening, and maybe factor it into future forecasts. Of course one should never assume that a major storm will weaken right before landfall - better to be safe than sorry, after all - but if meteorologists want to keep the public's faith in them high, I believe they are going to have to come up with some sort of explanation for why this keeps happening. Just repeating the mantra of "random fluctuations" is not going to be enough, I suspect.
 
Lately we've seen an awful lot of storms that have weakened significantly immediately before landfall, much more than chance alone can account for, or so it seems to me. I think there may be something else at work here, perhaps several somethings all contributing together, above and beyond the usual ERC fluctuations one expects to find in a major huricane at sea. We are going to have to understand this pre-landfall burst of weakening, and maybe factor it into future forecasts. Of course one should never assume that a major storm will weaken right before landfall - better to be safe than sorry, after all - but if meteorologists want to keep the public's faith in them high, I believe they are going to have to come up with some sort of explanation for why this keeps happening. Just repeating the mantra of "random fluctuations" is not going to be enough, I suspect.
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I agree.

People are just going to lose faith in them.
 
Normally 918 millibars would support a category 5. Still a very intense storm! Third strongest ever to hit the US in recorded history! Reportedly a 37 foot storm surge in Gulfport. For comparison, the highest surges ever recorded in the world have been 40-42 feet. Gulfport wasn't even in the eyewall. Pass Christian must be devastated!!!
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Wow! That is impressive and frightening. I don't have a good feeling about the death toll.
 
What worries me is that with all the emphasis on New Orleans, Mississippi coastal residents might not have evacuated as they should have.
 
Normally 918 millibars would support a category 5. Still a very intense storm! Third strongest ever to hit the US in recorded history! Reportedly a 37 foot storm surge in Gulfport. For comparison, the highest surges ever recorded in the world have been 40-42 feet. Gulfport wasn't even in the eyewall. Pass Christian must be devastated!!!
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Wow! That is impressive and frightening. I don't have a good feeling about the death toll.
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It is, most storms do weaken prior to landfall with interactions with shear/dry air and less depth to the warm water and ERC. Hurricane Katrina had a huge area of CAT 1 winds extending nearly 100 miles outwards compare that with Hurricane Dennis which barely had a small area of CAT 1 winds and a ridiclously small area of CAT 2- <3 winds. I think we will see some devastation from this storm and damages above 10 billion when all is said and done.
 
May I ask something...I am looking at IR satellite and an eye is still well visible. How long can it last afterall? Isn't this a bit rare thing, that the center is already pretty much inland, but the eye (plus eyewall) is still well organized. Thanks!
 
It possibly seemed to be wanting to intensify just when it hit? Landfalling intensifying hurricanes can stay organized longer over land. Good example is Charley, which retained his inner wind core the entire width of Florida.
 
Andrew said...
"So your saying that, when tropical storms/hurricanes, encounter shear or a dry air slot they start to weaken? I thougt shear what actually make it stronger. "

Hurricanes and extratropical cyclones(and the storms associated with them) are very different. Vertical wind shear is good for storms that develop with a midlatitude cyclone, but strong vertical shear is not good for a hurricane. Hurricanes form/strengthen in environments with little or no shear. As far as dry air goes, it does help to strengthen midlatitude storms when it lies over a layer of moist air near the surface. Hurricanes need a deep layer of moist air throughout the troposhere though.
 
I think two factors worked to weaken Katrina and possibly a third factor entered into play as well.

The first thing I saw yesterday evening was the increasing SW shear ahead of the developing trough over Texas. It doesn't take much shear at all to weaken a major hurricane as this one. From what I've studied, you've got to have absolutely perfect conditions aloft (anticyclone with zero shear) to support a cane like this. Those conditions did exist when it exploded to a major cat 5. But, as I said, the SW shear started impacting Katrina late yesterday which could be seen on WV loop as well as the IR analysis of the symmetry of Katrina getting contorted in a northward fashion.

The second thing as several people have already pointed out was some dry air entrainment. I've seen this happen with several hurricanes in the past couple of years now as they draw in drier air from the central CONUS.

The third thing I theorize is that it encountered some cooler water churned up by Katrina's vanguard effects. She was a slow moving hurricane with a rather large areal impact of winds and waves. I certainly think it is a reasonable assumption that the seas to the north of Katrina's eye was agitated for an extended period of time causing some cooler water below the surface layer to well up and mix thus creating some slightly cooler sea surface temperatures...especially near to the coast. I'd like to see some scientific studies and data to support this idea though, but certainly seems plausible from this amateur's perspective. ;-)

That's my $0.02 anyway.
 
I think two factors worked to weaken Katrina and possibly a third factor entered into play as well.

The first thing I saw yesterday evening was the increasing SW shear ahead of the developing trough over Texas. It doesn't take much shear at all to weaken a major hurricane as this one. From what I've studied, you've got to have absolutely perfect conditions aloft (anticyclone with zero shear) to support a cane like this. Those conditions did exist when it exploded to a major cat 5. But, as I said, the SW shear started impacting Katrina late yesterday which could be seen on WV loop as well as the IR analysis of the symmetry of Katrina getting contorted in a northward fashion.

The second thing as several people have already pointed out was some dry air entrainment. I've seen this happen with several hurricanes in the past couple of years now as they draw in drier air from the central CONUS.

The third thing I theorize is that it encountered some cooler water churned up by Katrina's vanguard effects. She was a slow moving hurricane with a rather large areal impact of winds and waves. I certainly think it is a reasonable assumption that the seas to the north of Katrina's eye was agitated for an extended period of time causing some cooler water below the surface layer to well up and mix thus creating some slightly cooler sea surface temperatures...especially near to the coast. I'd like to see some scientific studies and data to support this idea though, but certainly seems plausible from this amateur's perspective. ;-)

That's my $0.02 anyway.
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No ... that's a good assessment ... I was looking over a lot of the different models yesterday and I questioned the strength of the storm during the early morning hours when models were suggesting a moderate acceleration at the 500 mb level on upwards ahead of the trough over TX ... winds were up to 40 knots over eastern TX and western LA ... and via simple momentum propagation there was bound to be some means of shear affecting the storm strucutre ...

Overall I would suggest that such wind propagation and the dry air present between the two low pressure centers (the trough and HK) led to the demise of HK and her eventual weakening
 
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.
 
Didn't they categorize Hurricane Audrey as a cat 4 mainly because of the storm surge, with winds and pressure not quite reaching cat 4 criteria? I wonder how the NHC will categorize Katrina, with her pressure and surge supportive of cat 5 status?
 
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! :wink:
 
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