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HURRICANE KATRINA LANDFALL INTENSITY

By: ncforecaster, 2:49 AM GMT on May 27, 2008

Hey everyone,

First, I want to say that, "I hope each one of you have had a wonderful Holiday weekend!":) In addition, I want to personally thank Rays, Damon, Alex, Hurigo, Stormdude, Sandi, Fshhead (Jeff), Sullivanweather, and Suwanee for taking the time to post in my previous blog, and leaving me such thoughtful comments.:) If I am not mistaken, this will be the fourth blog entry I have written regarding this particular subject. Although I haven't had much time to post on these blogs during the past year or so, I have continued my own personal research into just how intense Hurricane Katrina's Maximum sustained winds (MSW) were at each of her Gulf Coast landfalls, respectively. In doing so, I thought some of you might be interested in "what I have found", as well as my own personal detailed synopsis on this rather controversial subject.

HOW STRONG WERE HURRICANE KATRINA'S WINDS AT LANDFALL?

In the paragraphs that follow, I will present the emperical evidence that exists, along with my own deductive reasoning (i.e. detailed analysis) as to why I personally believe that Hurricane Katrina came ashore as a 115-120 knot (132-138 mph) category four hurricane, very near Buras, La. at 6:10 am CDT on August 29, 2005. In doing so, I will also explain why I believe Katrina still retained the operational 110 knot (127 mph) intensity (strong category three/borderline category Four), when it came ashore once again, along the extreme SW Mississippi coastline, less than four hours later (9:45 am CDT).

IMPORTANT FACTORS TO CONSIDER IN REANALYZING "KATRINA":

1) Hurricane Katrina was a "double eyewall" storm at each respective landfall (making it far more problamatic in determing a definitive maximum sustained wind, as opposed to a single eyewall storm with a similar RMW).

2) The vast majority of anemometers in her path were either destroyed or disabled, well in advance of the arrival of the peak conditions.

3) It is common knowledge in the field that it is very rare for the maximum sustained winds to be recorded by land based anemometers (As a result, there is usually a rather large disparity between these measurements, and the official estimated sustained winds attributed to these storms in the post storm analysis (i.e. TCR).

4) Despite the "ragged" appearance of the "inner" eyewall (eye open to the SW), at the SE La. landfall, there is emperical evidence to suggest that the maximum sustained winds had been transferred to the "outer" eyewall, which didn't get sampled by Recon, as Katrina made its first Gulf Coast landfall.

5) Although Hurricane Katrina was certainly weakening as she was approaching the extreme SE La. shoreline, it is conceivable (if not probable) that she was NOT weakening "rapidly", as might be suggested by the "ragged" appearance of the inner eyewall (despite increasing vertical wind shear from an approaching upper level trough to its NW, apparent entrainment of dry air, and slightly cooler SST closer to shore). Instead, it appears that hurricane Katrina was weakening more "gradually", with the ERC having been aborted prematurely, so that Katrina had two significant wind maximums (consistant with the prospect that the overall weakening of her circulation was mitigated, and she didn't weaken as "rapidly" as determined in the TCR).

6) Despite the fact that there is not a direct one to one relationship between a given Barometric Pressure (B.P.) and the corresponding maximum sustained wind speed in hurricanes, the probable more "gradually" weakening eyewall winds (for the aforementioned reasonings), is more likely with Katrina's extremely low B.P at landfall (920 mb). Keeping in mind, my best educated GUESS means that a 920 mb pressure is still VERY LOW for a 115-120 Knot (132-138 mph) minimal to moderate category Four storm. Regardless of the actual maximum sustained winds that existed at landfall, Katrinas gradually rising B.P. in association with Katrina's expanding wind field (i.e. RMW) decreased the "steepness" of the pressure gradient, causing her winds to "gradually" reduce in intensity. That being said, that fact alone doesn't support such a significant variation between B.P. and corresponding wind velocities.

Important note: Wind is simply air in motion, and its velocity is determined by the attendant pressure gradient, which is simply the differences in pressure over a given distance. Therefore, the greater the difference in pressure that exists for the same distance, the stronger the wind velocity will be. This is the reason hurricane Andrew was able to generate 145 knot (167 mph) maximum sustained winds (category five on the SSHS), with a corresponding lowest B.P. of 922 mb (strong category four on the SSHS). Moreover, it is the extreme differences in pressure over such an extraordinarily short distance that helps tornadoes generate such violent wind speeds.

7) There were some very impressive wind gusts recorded well inland (prior to these anemometers being destroyed and/or disabled before they could measure peak gusts) that caused widespread and extensive "wind" damage as far as 150 miles inland. As I stated previously, "ground truth" post storm damage surveys are generally a very good indicator of just how intense thw winds might have been for a given locality. The Jackson, Mississippi NWS conducted these surveys immediately following Katrina's destructive course through their designated warning area. They concluded that the most significant damage bordered on F3 intensity (the NWS uses the fujita tornado intensity scale to categorize damage for a given area). This data combined with detailed analysis of Katrinas apparent rate of weakening as she pushed further inland from the coastline (by analyzing the corresponding radar imagery and radial velocities, etc.), provides one with important information in attempting to extrapolate the estimated winds back to the point of each respective landfall.

Note: The Jackson NWS listed the following inland wind measurements for their CWA (Centralized Warning Area), all taken prior to the anemometers being destroyed and/or disabled.

a) Popularville (45 miles inland) 117 knots (135 mph) Actually, from the New Orleans/Baton Rugue, La. NWS CWA

b) Ellisville (90 miles inland) 99 knots (114 mph)

Note: 70 knot (81 mph) sustained 2 minute winds were recorded, before the anemometer was destroyed by the strong winds.

c) Laurel (110 miles inland) 96 knots (110 mph)

d) Hattiesburg (65 miles inland) 87 knots (100 mph)

Note: This reading was taken from the Forrest County EOC in South Hattiesburg (5-7 miles SSW of Hattiesburg) at 1 PM CDT, a full 30-45 minutes prior to the arrival of the peak wind gusts. Ironically, I was filming the action at a location only a mere 1/10th of a mile away and that was the time the peak winds impacted the area. Based on my own experience (conducting damage surveys in the past), I estimated the peak wind gusts in the 100-109 knot (115-125 mph) range (more importantly, this is consisent with the Jackson NWS estimates per our correspondence). There were/are other surveys conducted by other agencies suggesting wind gusts may have even reached 114 knots (131 mph). Although, I would suggest these estimates are most likely 5-9 knots (5-10 mph) too high, in my own humble opinion. Interestingly, the HRD HWIND analysis places estimated maximum sustained winds at 84 knots (97 mph) at 1800 UTC (1 PM CDT) with eye itself centered less than 20 miles SSW of Hattiesburg (which is located 70-75 miles inland).

Jackson, MS. NWS Post Storm Report

8) Examination of radial velocities and relectivity from the Slidell, La. NEXRAD radar suggests that the operational landfall intensity of 120 knots (138 mph) may very well have been correct for Katrina's first Gulf Coast landfall.

AMS Paper by Campo, et. al, for Katrina's Radar Analysis

9) The TCR left open the prospect that sustained category four winds did in fact impact the extreme SE portion of La. This apparent "disclaimer" so to speak by the TCR research forecasters, as well as the fact that Katrina wasn't identified as a "double eyewall" hurricane until after the TCR was published, increases the probability that hurricane katrina still retained maximum sustained winds of at least 114 knots (or 131 mph, which is the minimum threshold for category four intensity on the SSHS) when its eye intersected the SE La. shoreline near Buras, at 6:10 am CDT (as opposed to 110 knots/127 mph).

NEW RESEARCH PAPER/REPORT ON KATRINA'S LANDFALL INTENSITY:

While I was browsing the Web so I could copy and paste some of the "Links" highlighting the various sources, reports, and data I've discovered during my own personal research on this subject matter (during the past year and a half), I stumbled across this "NEW" AMS (American Meterological Society) paper authored by "Richard Henning". In it, He details alot of the same relevant information I was presenting in the aforementioned paragraphs. Although I quickly skimmed through it just now, he appears to be suggesting that the intense convection associated with the NE portion of the eyewall actually brought category four sustained winds '(115 knots (132 mph), gusts to 140 knots (161 mph)' to the hardest hit areas along the Mississippi coastline (i.e. Waveland, Bay St. Louis, and Pass Christian). As time allows, I will be most interested in reading through this report far more thoroughly, and see if I personally concurr with his reasoning (based on the emperical evidence, etc.).

AMS Paper by "Henning" 2008

Note: He was the author of one of the reports I had linked in a previous blog entry on this subject, where he discussed the correlation between deep convection and strong wind gusts. In it, he explains how the highest winds that are found in the mean boundary layer can be transported to the surface through deep convective thunderstorms (heights exceeding 50 dbz on radar reflectivity) associated with the eyewall. Here is that link once again, if you're interested in reading it as well.

AMS Paper by "Henning" 2006

IN CONCLUSION:

While I had planned on presenting more of my own personal synopsis, I believe I have already presented enough evidence to support my own personal conclusions that Hurricane Katrina was still a 115-120 knot (132-138 mph) category FOUR at its first Gulf Coast landfall. Likewise, I believe the aforementioned evidence (detailed throughout this post) also strongly suggests that the operational intensity of 110 knots (127 mph) was indeed correct for hurricane Katrina's second Gulf Coast landfall in extreme SW Mississippi. Regardless, there is still enough uncertainty that exists in this most inherently complex, and rather subjective endeavor (i.e. attempting to place a definitive "Maximum sustained wind speed" for each respective landfall), that it is most probable that we will never truly know just how intense the MSW were, when this historic and incredibly catatrophic storm devastated the North Central Gulf Coast, on August 29, 2005. Thanks so much for taking so much time to read this lengthy post, and I hope each one of you has a great rest of the night, and a good start to the week to come.:)

Personal note: Since any community such as this will consist of those with varying degrees of knowledge relative to the science of tropical meteorology, some of the information I displayed may have seemed unnecessary and/or quite redundant to some. On the other hand, I would rather error on the side of being too descriptive for lack of a better word, since I personally believe we each have a unique opportunity during such discussions to help share some degree of knowledge and/or experience, that could possibly benefit others. With that in mind, I am so thankful for all those mentors in my own life who've been so willing to invest their time, knowledge, and experiences in helping me continue to develop an even greater understanding and appreciation for the science. Moreover, this forum has consistantly reaffirmed my belief (thanks to all the wonderful and knowledgable people I've met here during the past 2 1/2 years) that we can all learn from each other, through our own unique education and/or experiience (whether it was obtained by working directly in the field or otherwise).

Most sincerely,
Tony

Updated: 8:17 AM GMT on May 27, 2008

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