WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

Katrina is gone

By: Dr. Jeff Masters, 6:56 PM GMT on August 31, 2005

Katrina lost its tropical characteristics this morning and is no longer a tropical depression, just a regular low presssure system. She continues to dump 2 - 4 inches of rain along her path, and is a threat to bring minor flooding to northern New England today and tomorrow. Katrina-spawned tornadoes killed two in Georgia and damaged 13 homes in Marshall, Virginia yesterday, but the threat of tornadoes has diminished today. Katrina is no more, a blessing we can use right now!

Blessings are very hard to find in the catastrophe zones of Louisiana and Mississippi today. We are only just now beginning to hear from the areas that the calm of the eye passed over. Slidell, on the western side of the eye's passage, received a 15-foot storm surge. Only foundations are left of a large portion of the buildings, and the bridge to New Orleans is broken in multiple places, according to news reports. Pass Christian, unlucky holders of the U.S. record-highest storm surge of 24.7 feet from 1969's Hurricane Camille, received a 20-foot storm surge. The storm surge at nearby Bay St. Louis was 22 feet, and both areas have extensive areas of complete destruction.

Why did the New Orleans flood walls fail?
The 325-mile long series of flood walls and levees surrounding New Orleans were engineered to withstand the storm surge from a Category 3 hurricane. No Category 3 or higher hurricane has hit New Orleans in the past 150 years, a strange quirk one would not expect based on the pattern of hurricane strikes elsewhere along the Gulf Coast. New Orleans should get a Category 3 hurricane passing within 80 miles every 32 years, a Category 4 hurricane every 70 years, and a Category 5 hurricane every 180 years. However, the strongest hurricanes ever to hit the city were two Category 2 hurricanes--a 1893 hurricane that killed 2000, and Hurricane Betsy of 1965, which killed 75 and put parts of the city under 8 feet of water. Hurricane Camille, although it was a Category 5 hurricane and took almost the same track as Katrina, was a very small hurricane with hurricane force winds extending out only about 50 miles from the center. Camille brought 100 mph gusts to the eastern side of New Orleans. Katrina was a huge storm whose hurricane force winds extended a full 110 miles from the center, and probably brought 130 mph wind gusts to the same area. Katrina piled up a much larger storm surge wave onto the flood walls than Camille. According to the Army Corps of Engineers, "What failed were actually floodwalls, not levees. This was caused by overtopping which caused scouring, or an eating away of the earthen support, which then basically undermined the wall. These walls and levees were designed to withstand a fast moving category 3 hurricane. Katrina was a strong 4 at landfall, and conditions exceeded the design." The flood wall breaks lie along Lake Pontchartrain, whose water is 4.5 feet above sea level. Thus, since New Orleans lies 6 feet below sea level, we can expect the city to flood to a depth of at least 10 feet.

I highly recommend reading an October 2001 article from Scientific American, titled: Drowning New Orleans, to learn more about the vulnerabilities of the levee system.

The tropics today
The most significant threat in the tropics I can see is the potential this weekend for a tropical depression to develop in the coastal waters surrounding Florida. This is the same location that Katrina developed. However, this time the development might come at the tail end of a cold front that is expected to push off of the East Coast, instead of from a tropical wave. If a depression does form in this area, the possible track is impossible to guess at this point.

Tropical Depression 13 is nothing to worry about, as it is over the open Atlantic Ocean and heading out to sea. The well-organized wave 1300 miles east of the Lesser Antilles will probably become Tropical Depression 14 in the next day or two, but it is probably too far north to threaten any land areas. This system will probably recurve out to sea. The tropical wave that pushed off of the coast of Africa yesterday and is now south of the Cape Verdes Islands has some potential for development later in the week, and the GFS model projects that this wave will become a hurricane and a potential threat to the Leeward Islands a week or so from now.

On August 19, I posted the image below showing a long-range forecast for August 31 from the GFS model.

As you recall, mid-August was a time of relative quiet in the tropics, but the GFS model was calling for an end to this quiet period. The 12-day GFS forecast called for 3 tropical cyclones for August 31. Well, the GFS was correct in calling for an end to the quiet period! While there is only one tropical cyclone (TD 13) out there, the other two strong tropical waves seen in the satellite image above certainly have the potential to become tropical depressions over the next few days. The GFS did miss the formation of Katrina, but the general 12 day forecast showing a big increase in hurricane activity was pretty accurate.

Dr. Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.