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!

 

[Chris Strahan]

from what I saw, I think one of the main weakening factors is that the storm began sucking in dry air from the mainland.

 

[SummerStar]

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.

 

[Karen Politte]

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

 

[Twisterdave]

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.

 

[Jeff Snyder]

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).

 

[Mike Peregrine]

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.

 

[SummerStar]

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?

 

[Jeff Snyder]

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.

 

[Scott Olson]

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.

 

[Scott Olson]

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.

 

[Edward Ballou]

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.

 

[SummerStar]

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?

 

[Andrew Khan]

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.

 

[Michael Auker]

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!!!