Katrina Winds

[Chris Sokol]

Can anybody verify this? If so, this would indicate Katrina was actually a Catagory 5 at landfall. It is from the City of Slidell website.

The Weather Service reports that Slidell had sustained winds of 176 mph and gusts of 190+ mph during Hurricane Katrina. In addition, Slidell was hit by a 23' - 26' storm surge that devastated much of the city. This has been very devastating for everyone, but we are making great progress thanks to the many city workers, police officers, firefighters, military troops, citizens and volunteers who have worked so hard these last few weeks.

http://www.slidell.la.us/

Note: This information does not verify with anything that I am able to find on the New Orleans WFO website.

 

[J West]

Also from the Slidell website:

Radio Slidell at WOPR 94.7 FM.

That's a blast from the past (no pun intended ) It went from chewing on ICBM launch codes and playing tic-tac-toe with itself to being a radio station :wink:

Back on topic...wouldn't the NWS offices down there have something about the wind speeds posted on their sites?

 

[B Ozanne]

Same topic, different location. I heard an industry spokesperson on the news say that an oil rig/platform had sustained winds in the 170's with gusts over 200mph. This was just yesterday they came out with this. Who knows, I'm guessing the rig was near Katrina when it had winds of that speed so they just assume the rig had those winds.

I think its nothing, but if there is any truth to it we'll know in the next few days.

I was watching the Slidell radar during the storm and I could swear there was a cell moving at/around 130kts just about to hit the site before it went offline. Does anybody remember this? Maybe there is something behind that.

 

[Aaron Kennedy]

I had heard a max of 130mpg gust at the NWSFO in Slidell....

Aaron

 

[rdale]

New data suggest Katrina was a less intense, Cat 3 storm

http://tinyurl.com/7v7cl

"New, preliminary information, compiled by hurricane researchers, suggests the system struck southeast Louisiana on Aug. 29 with peak-sustained winds of 115 mph. That would have made it a Category 3 storm, still a major hurricane but a step down from the enormous destructive force of a Category 4.

Katrina might have further downgraded to a strong Category 1 system with 95-mph winds, when it punched water through New Orleans' levees, severely flooding most of the city and killing hundreds. The levees were designed to withstand a Category 3 storm."

 

[cdcollura]

Still very dangerous - regardless

Good day everyone,

As you know, whether or not Katrina had 115 MPH or 155 MPH SURFACE winds as it tore into the coast, it DID have a CATEGORY 5 storm surge (over 30 feet in some places) and a CATEGORY 5 central pressure (915 MB).

Much of this is under investigation, I even came up with a theory about weakening systems still having a high storm surge since the "dome" of water and waves (mass transport plus translation) atop it have not "winded down" yet, in response to the lesser winds. This theory explains the high surge, but does not explain the pressure of 915 MB without the category-5 winds expected.

My storm surge height vs winds theory is available at the link to another topic below (very interesting read)...

http://stormtrack.org/forum/viewtopic.php?t=8686

It is also possible that winds just above the ground could have been MUCH stronger, possibly strong 4 (or even 5) force, but not "mixed down" since a cooler layer developed in the boundary layer air, especially as the storm was nearing or over land.

 

[Dave Lewison]

I've actually heard this theory before and it does make sense. Water has such an immense inertia that it takes time for any changes in wind to manifest in the water height. I've heard that it takes at least 24 hours for a storm surge to respond to the level of the hurricane that created it. With Katrina, you had a CAT-5 storm at full strength for an entire day, and then weaken to CAT3 in less than 12 hours. That will build up a tremendous amount of water, which takes time to dissipate. Since it hit land so quickly thereafter, the surge was still near peak levels. With Rita, the storm was on a slow downward trend for the whole day preceding landfall. That's one reason I think the surge was not quite as bad as with Katrina.

Now as for pressure, I don't believe that's an accurate indication of expected windspeed. It's the pressure GRADIENT that creates the wind. The tighter the gradient, the faster the wind. A weakening hurricane such as Katrina would tend to spread its pressure gradient and windfield out laterally, maintaining the core low pressure for a while, but reducing the top windspeeds. I've always wondered if this applies to tornadoes too...the thin "drill bit" tornadoes look so much more violent than their larger counterparts.

 

[HAltschule]

Alright...this will sound snobby but I really do not intend it to be:

A. I don't get it? Why is this a new theory? This is already well known by Meteorologists..especially Marine Forecasters. It is known that there is a delay from the time the winds subside to the time the storm surge decreases. Can you elaborate on what is "new" about it?

B. I highly doubt that an inversion would have had an affect on the maximum winds translating down to the surface. A tropical system is a warm core system and very likely would not have had an inversion to that magnitude. The key to the weakening of Katrina was likely directly related to any or all of the following: 1. Cooler eddy's of water just before landfall; 2. Eyewall replacement Cycle, or 3. Normal convective bursts and lulls of activity that are common in massive storms. These are all very common, reasonable reasons for the slight weakening before landfall. I don't beleive a classic inversion was present and certainly not to the magnitude where it would prevent the strongest winds from reaching the ground.

 

[cdcollura]

Good day, and thanks for your input too,

Maybe my wording about a "new" theory may be ambiguous. I see forecasts saying the winds weakened therefore less surge could be expected. Obviously, "less surge" did not happen with Katrina. If meteorologists are already recognizing the interia of the storm surge, well that is great.

I just wanted to bring this to the table and get as many opinions as possible from you guys on what you think about it. The possibility of an eyewall replacement is more so than an "inversion". Air, however, does cool slightly at the surface in a landfalling storm due to evaporative cooling or rain, and the fact the ground does not have much in the way of heat content compared to the ocean (that's why caverns are 50 degrees year round).

There could have been many other factors, most likely dry air entrainment, cooler water near the coast, even slight wind shear aloft that weakened the storm. I also noticed that MOST nothern Gulf hurricanes do this when they come ashore, with the weakening ocurring: Opal 1995, Lili 2002, Ivan 2004, Dennis 2005, Katrina 2005, and Rita in 2005.

 

[rdale]

Since RH's are pretty much at 100%, I can't believe that there would be anywhere enough cooling for a disconnect inversion at the surface. Turbulence on its own would be more than ample to get the high winds down there...

 

[HAltschule]

Agreed. Keep in Mind, Chris, that evaporative cooling occurs when there is dry air near the ground. In these cases, the air is saturated and the precip is falling into an already saturated column.

From my recollection of Katrina, there was a lot of dry air that came in on the south and eastern side of the storm. As indicated in the previous post, it's amazing how a little dry air can cause such a decrease in winds.

 

[Glen Romine]

I'm definitely not an oceanographer - so storm surge is not something I know much about. The pressure effect though is a pretty minimal contributor to storm surge (100 mb pressure deficit in a hurricane is equivalent to 1 m [3 ft] storm surge). So, for the ~32 foot peak storm surge noted with Katrina, we have to be talking about influences from wind driven waves and how these lead to water accumulation along the coastline. In the deep ocean the water within waves just oscillates in a circle - only the wave energy is transported, not the water. The bigger the wave - the faster the motion, so the largest waves (swells) quickly leave the hurricane and transport energy well away. However, in shallow water this is not the case, and can lead to water transport as the wave energy is dissipated at the coastline. It's known the bathymetry is important (how the transition from deep ocean waves to shallow water waves) as well as the shape of the coastline (concave coastlines are less efficient at allowing horizontal transport of the accumulated water). We know Katrina was an unusually large hurricane. Perhaps that offered a longer fetch of moderately high winds which was more important for building and maintaining large waves than a small area of higher winds would have been.

Definitely more knowledgeable storm surge folks out there, but I think the simplified storm surge interpretation passed on to us laymen has led to some significant misuse of the standard surge height relationship with hurricane intensity.

Glen

 

[MClarkson]

katrina also came in directly perpendicular to the coast for quite some time. This allows a long and uninterupted fetch on the right side... also considering the immense intense windfield, topography of the gulf, its basically near perfect conditions for surge. A slight weakening just before landfall isnt going to change much.