NHC: Katrina Cat 3 at landfall, not 4

[Jeff Snyder]

NHC published it's final report on Katrina, and it has adjusted the intensity of Katrina downward on Louisiana landfall.

The aircraft data from 29 August indicate that the structural changes in Katrina were associated with its rapid weakening to a high-end Category 3 hurricane just before landfall in Louisiana. The strongest surface (10 m) wind measured by dropwindsonde on the morning of 29 August was 99 kt from two separate sondes. The maximum surface wind estimate from a dropwindsonde, derived from the mean wind over the lowest 150 m of the sounding using an 7 average adjustment derived from profiles in several storms, was 98 kt. However, analysis of several dropwindsonde profiles from 29 August suggests that a slightly different adjustment could have been valid that day. This difference would result in 10 m wind estimates derived from the lowest 150 m of the dropwindsonde profiles being 3-5 kt stronger, or up to about 103 kt. The maximum surface wind measured by the SFMR on 29 August was 96 kt just after 1200 UTC. The best track intensity of Katrina at 1200 UTC 29 August, shortly after the initial Louisiana landfall when the central pressure was 923 mb, has been adjusted downward in poststorm analysis to 110 kt from the operationally assessed value of 120 kt. The Buras, LA landfall intensity about one hour earlier has also been estimated at 110 kt, when the central pressure was only slightly lower at 920 mb. This estimate is still about 10% greater than the maximum surface winds from the dropwindsondes and SFMR, accounting for the possibility that these instruments did not sample the maximum wind. It is worth noting that Katrina was likely at Category 4 strength with maximum sustained winds of about 115 kt near 0900 UTC 29 August, a couple of hours before the center made landfall near Buras, LA. Due to the large (~25-30 n mi) radius of maximum winds, it is possible that sustained winds of Category 4 strength briefly impacted the extreme southeastern tip of Louisiana in advance of landfall of the center.

The report also notes that Katrina had the lowest pressure observed with a 110kt hurricane, surpassing Floyd.

The strongest sustained wind in New Orleans is subject to
speculation since observations are sparse, due in part to the power failures that disabled ASOS stations in the area before peak wind conditions occurred. However, the NASA Michoud Assembly Facility in eastern New Orleans measured a 1-minute sustained wind of 84 kt (at an elevation of about 12 m) near 1100 UTC 29 August. Also, a few instrumented towers placed in various locations in the metropolitan area by the Florida Coastal Monitoring Program (FCMP) and by Texas Tech University measured sustained winds in the range of 61-68 kt. The Mid-Lake Pontchartrain NWS site (16 m elevation), located along the Lake Pontchartrain Causeway about 8 n mi north of the south shore of the lake, also measured a one-minute sustained wind of 68 kt. Even though these various sites likely did not experience the maximum wind in the area, the Mid-Lake Pontchartrain site had open marine exposure, unlike most locations in the city of New Orleans. It appears likely that most of the city experienced sustained surface winds of Category 1 or Category 2 strength.

Initial total damage (insured+uninsured) estimate: $75 billion, with a strong disclaimer that final costs may be considerably more.

Max surge height: At least 27', with strong difficulty in determining max surge height at areas near the eyewall owing to significant damage.

View the final report at http://www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf

 

[Karen Politte]

Sounds consistent with what we already know. I believe Tim Marshall (AKA the Gulf Coast snowbird :wink: ) only found evidence of Cat. 3 wind damage also, during his extensive and ongoing damage surveys of the area. The water damage from Katrina was clearly the headline-maker.

KR

 

[Robert Edmonds]

That's what I was thinking when it made land fall that it was 3 but everyone was saying it was a four at the time. I think this could have some implications because the levy system in NO I believe was suppose to be good for a cat 3.

 

[Gabe Garfield]

That's what I was thinking when it made land fall that it was 3 but everyone was saying it was a four at the time. I think this could have some implications because the levy system in NO I believe was suppose to be good for a cat 3.

The levy breaches in N.O. were very surprising, but I still think that the Gulf Coast was affected by a much higher storm surge than a typical Category 3 storm. Katrina was a Category 5 storm only a half a day before she made landfall, so the storm surge that built up did not have enough time to subside, which is why places like Gulf Port and Biloxi had such horrific destruction. So, while Katrina was a Cat. 3 at landfall, her storm surge was more typical of a Category 5 storm.

Gabe

 

[Pat Lawrence]

Thanks for the info, Jeff. I think it was either you or Aaron that had some doubts about it being a cat 4 at landfall to begin with.

In any event, the NHC surely, finally, assigned the appropriate rating/ranking/severity.

Pat

 

[cdcollura]

A Cat-3 storm with a Cat-5 surge!

Good day everyone,

My group (Weathervine) and I chased Katrina and were stationed along the coast on Highway 90 1/4 mile from the Gulf at the Mississippi Coast Coliseum between Gulfport and Biloxi in Harrison county. The lobby of this place is at least 20-25 feet above sea level, high enough for a category-3 surge, or so we thought ;-(

I would agree and conclude with the winds at near 130 MPH at landfall (high end cat-3) ... As the wind damage observed, once you got a bit inland away from the surge, ofcourse, was much like category-3 damage, not much more.

Now, the big KICKER was the STORM SURGE in Katrina. The storm weakened due to the combination of eyewall replacement cycles (restructuring of hurricane core region) as well as the ingestion of some drier air from its northwest.

The dry air eroded the eyewall of Katrina on its southern and western side and allowed for some weakening, in addition to the weakening cycle of eyewall replacement that has not completed yet. Like a spinning ice skater bringing OUT her arms, Katrina also slowed down as the storm widened. This is why the pressure was so low at 915 to 920 mb (more like a strong cat-5) but had cat-3 winds over a WIDE area.

The storm surge was EXTREMELY high in Katrina, and the worst of it did not even hit New Orleans! The combination of residually higher surge (generated when the hurricane WAS stronger - as a surge is a wave and takes time to "wind down") and longer fetch (due to a larger storm) produced the highest storm surge in history - More like that of a strong cat-5 - Over 30 feet!

I saw floating cars in the lobby and crashing into gates and 5 feet of water in the first floor of the coliseum and had to go to the next level. My car was spared from the salt water by driving a ways it up a flight of stairs on the outside!

The storm may have been weaker, and I do agree with that, but that is not very comforting because the WIND was not the problem, it was the WATER, and earlier statistics show that 90% of hurricane deaths are from drowning and water related issues.

A good analogy to the weakening of Katrina: Kind of like being infected with the Ebola virus, which has a high fever and bleeding to death, but having a type of Ebola without the FEVER but still all the other symptoms that will kill you! Not very encouraging.

Katrina may have had less wind, but still all the other things far more deadly than the wind ;-(

Chris C - KG4PJN

 

[rdale]

I think this goes to show that the xxx mb = yyy mph correlations might be going the way of 10:1 snowfall ratio or 10*C = capped assumptions we've always had in the past...

- Rob

 

[Justin Turcotte]

I think this goes to show that the xxx mb = yyy mph correlations might be going the way of 10:1 snowfall ratio or 10*C = capped assumptions we've always had in the past...

- Rob

None of the folks I work with nor myself are professional hurricane forecasters but we have noticed that smaller cyclones, especially those with small eyes tend to have winds underforecast by the TPC. Their rate and magnitude of intensification is often underforecast. The larger spawling storms seem to have winds overforecast by the TPC. This said, I always have to remind myself that the area of maximum winds in a hurricane is quite small and therefore instruments are not likely to capture those peak winds. Consider a wide F5 tornado: The area of F5 damage is usually very small. This area is bound by a fairly small radius of F3-F4 winds which is bound by a much larger footprint of F0-F2 winds.

 

[MClarkson]

I think the corellation is still generally valid if you do something like


xxx mb / size = yyy mph

 

[Jeff Snyder]

I think the corellation is still generally valid if you do something like


xxx mb / size = yyy mph

I'm not sure... you're essentially saying that PGF is proportional to surface wind speed... Theoretically, and ideally, this very well should be the case. However, as we've seen, even this isn't necessarily valid. Wilma had a very small area of hurricane force winds when she was uber-bombing to <900mb, yet she also "only" had 175mph winds. You can figure out a PGF force for a storm like Katrina while she was ~902mb, but I can almost guarantee that the same PGF-to-wind relationship wouldn't work well for Wilma, seeing how Wilma had lower pressure AND a smaller wind field (in and immediately following her rapid intensitification phase), yet wasn't that much stronger in terms of surface winds. We've also seen hurricanes that have expanded while maintaining a relatively steady central pressure yet also quasi-steady max winds. So, I'd say that PGF, while obviously important in driving the winds, is not always an indicator of winds in hurricanes, particularly for those that are weakening or strengthening. Yes, a storm that is 200mi wide with a CP of 910mb will have stronger winds than one that is the same size yet with a CP of 990mb, but I wouldn't bet much money in determining the winds (even comparatively) of a 910mb vs. 920mb storm. There's just a lot that isn't known (e.g. the 90% reduction rule works well for some storms, yet doesn't work well with others).

 

[MClarkson]

I trust the "standard" flight level reductions least of all. They seem to be in error more often than the other generalizations that can be made.


The thing with Wilma is that she had a very tight core of intense winds but she had a large swath of tropical storm force winds, so comparing her to an "average" tropical cyclone is troublesome, given that one size parameter is below average and one is above average... and I heard a rumor that she is gonna get bumped up to 185 mph durring NHCs final report. Some news site had a picture of the official NHC hurricane chart with wilma at 185... could have been a mistake I dont know.

an Ivan or Katrina is larger than average in all respects, an Andrew or Charley smaller in all respects, its storms like that where I think mb/size=wind should be reasonably close.