Please analyze/explain why/how Katrina weakened

Curious Guest

It seemed yesterday that all indicators favored a category 5 landfall.
I cannot recall hearing one report of potential weakening mainly due to the warm waters.

Can you guys take a stab at this. Why/how did it weaken just prior to landfall in LA? What factors came into play, etc...

Thanks!
 
Could it be that the forecasters wanted to err on the side of caution?

Or maybe that accuracy of forecasting is still far from perfect.
 
I'd be interested in speculation, also.

I've lost count of how many systems I have seen cranking in the Gulf. Then, when they get anywhere NEAR the coast of LA or MS - they start to crap out.

Katrina looked like she was starting to encounter shear in her NW quadrant as early as last night at 11.00pm.

Then, this morning, her whole NW side was a bit askew. Did she entrain dry air from land by this point?

If so - is a slow-moving storm like this ever going to hit at full intensity, if it can be adversely affected by merely its proximity to land? I wonder if it has something to do with the juxtaposition of the landforms in this particular area of the Gulf - as opposed to, say, FL or TX - which seem to have far more intensifying landfalling storms.

Weird stuff. I don't like tropical weather.

KR
 
RE: Weakening at landfall

There was a clear slot of dry air that moved into the eyewall around the time that it made landfall, and the shallower water cools down more as wave and wind action increases evaporation and has less volume.

Those factors have been shown to weaken a hurricane slightly.
 
Most hurricanes that attain Cat 4-5 status only hold that status for a short time. The conditions necessary for such as strong hurricane as quite rare, as one would expect. My hunch is that the storm was going through an ERC when dry air to its west finally payed a huge toll. A loop of the IR imagery through the early morning (after the GOES eclipse) showed that cloud tops cooled drastically over the western half of the storm. Dry air may have made its way to the inner eye, though other factors may have contributed to the ragged inner eye. Near landfall, the inner eye only ahd about 50-75% coverage (it was open from southwest through east some of the time). As I mentioned in another thread, there was a convective burst immediately at landfall, and cloud tops cooled quickly near the eye, so I do think that it was trying to restregthen. We may have seen a legit Cat 4-5 at landfall had this happened. The strongest wind I've heard was a 135mph gust near Slidell. Given that this was a gust, the sustained wind was probably in the Cat 3 range. I hate it when folks measure a one-second gust and use that speed to classify the hurricane. For example, someone measuring a 80mph gust and say that they have hurricane force winds. Max gusts do not indicate hurricane category as the wind speed thresholds are typically defined... Saffir-Simpson categories are based on sustained winds (>74mph sustained for Cat 1).
 
Storm cycling in hurricanes is normal - Cat 5 storms seldom stay at Cat 5 but at times cycle stronger and at times weaker. I believe we saw Katrina move through a particularly strong cycle during the afternoon hours yesterday and then simply cycle into a weaker phase as it was making landfall this morning. It was noted that dual eyewalls were attempting to form at times during the night, which can indicate storm cycling and weakening phases. If it was allowed to remain over that warm water it would have been possible for it to re-strengthen, but it moved northward over land, drawing in the continental air, which has a weakening affect on the storm.
 
I hate it when folks measure a one-second gust and use that speed to classify the hurricane. For example, someone measuring a 80mph gust and say that they have hurricane force winds. Max gusts do not indicate hurricane category as the wind speed thresholds are typically defined... Saffir-Simpson categories are based on sustained winds (>74mph sustained for Cat 1).

Dumb question, but what's the actual definition of a 'sustained' speed? For how long does it have to be measured?
 
I hate it when folks measure a one-second gust and use that speed to classify the hurricane. For example, someone measuring a 80mph gust and say that they have hurricane force winds. Max gusts do not indicate hurricane category as the wind speed thresholds are typically defined... Saffir-Simpson categories are based on sustained winds (>74mph sustained for Cat 1).

Dumb question, but what's the actual definition of a 'sustained' speed? For how long does it have to be measured?

From the Louisville NWSFO glossary:
"The wind speed obtained by averaging the observed values over a one minute period."

--> http://www.crh.noaa.gov/lmk/glossary.htm

Another source says that the World Meteorological Organization defines "sustained winds" to be averaged over a 10-minute period. Sustained winds tends to be signficantly less than peak gusts.
 
I hate it when folks measure a one-second gust and use that speed to classify the hurricane. For example, someone measuring a 80mph gust and say that they have hurricane force winds. Max gusts do not indicate hurricane category as the wind speed thresholds are typically defined... Saffir-Simpson categories are based on sustained winds (>74mph sustained for Cat 1).

Dumb question, but what's the actual definition of a 'sustained' speed? For how long does it have to be measured?

See above.
 
Even under the right conditions Hurricanes rarely stay under one intensity level for a long time. Im sure had more time been avaliable Katrina would have strengthend some more. Some dry air did contribute and just flucuations. After a intensifying trend to 902mb (yikes) theres no where else to go but down with such an historic intensity.

Still Katrina made landfall as an amazing Hurricane at 918mb and 110kts of anaylzed surface wind. I would have expected some of the highest windspeeds to have been recorded in the Plaquemines of LA.
 
Another contributing factor to the weakening before landfall is that the depth of water significantly decreased as Katrina came ashore. Perhaps you could call it a delta effect or something. The availability of warm water available could have choked off a sustaining flow.
 
This is a fascinating subject, but one I know next to nothing about! One more question related to this: Does solar radiation make any difference to a storm's ability to intensify, or would the sea temperature's constancy make this negligible? Will this factor have any effect on K as its remains track over the continent in the next few days?
 
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. I remember I saw it looked like a very well formed text book example of a hurricane formation. Perfect structure, classic. And then late last night, the Left side of the storm the west side, began to die away, and erode. This was a curious thing.
 
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!!!
 
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.

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!!!

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!!!

Wow! That is impressive and frightening. I don't have a good feeling about the death toll.

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.
 
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