Hurricane Katrina may have been a minimal Category 3, not a strong Category 4, when it made landfall — and it didn’t even produce sustained hurricane-force winds over all of Lake Pontchartrain, which nevertheless breached the New Orleans levees and flooded the city! Excerpt:

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 [as it moved due east of New Orleans]. …

In its original reports, the hurricane center reported Katrina struck near the town of Buras, about 55 miles south of New Orleans, with 145-mph winds and weakened to 125 mph when it was about 35 miles east of the city, all the while pounding the levees. …

Powell said the new data show that Katrina packed 95-mph winds over the east end of Lake Pontchartrain and about 65 mph over the west end, enough to cause the levees to fail.

Of course, that shouldn’t have been enough — not NEARLY enough — to cause the levees to fail. But then, we already knew that the wind-driven storm surge didn’t “overtop” the levees; rather, flawed levees doomed New Orleans:

Louisiana’s top hurricane experts have rejected the official explanations for the floodwall collapses that inundated much of New Orleans, concluding that Hurricane Katrina’s storm surges were much smaller than authorities have suggested and that the city’s flood- protection system should have kept most of the city dry.

The Army Corps of Engineers has said that Katrina was just too massive for a system that was not intended to protect the city from a storm greater than a Category 3 hurricane, and that the floodwall failures near Lake Pontchartrain were caused by extraordinary surges that overtopped the walls.

But with the help of complex computer models and stark visual evidence, scientists and engineers at Louisiana State University’s Hurricane Center have concluded that Katrina’s surges did not come close to overtopping those barriers. That would make faulty design, inadequate construction or some combination of the two the likely cause of the breaching of the floodwalls along the 17th Street and London Avenue canals — and the flooding of most of New Orleans.

In the weeks since Katrina drowned this low-lying city, there has been an intense focus on the chaotic government response to the flood. But Ivor van Heerden, the Hurricane Center’s deputy director, said the real scandal of Katrina is the “catastrophic structural failure” of barriers that should have handled the hurricane with relative ease.

“We are absolutely convinced that those floodwalls were never overtopped,” said van Heerden, who also runs LSU’s Center for the Study of Public Health Impacts of Hurricanes.

In an interview Tuesday, Corps spokesman Paul Johnston said the agency still believes that storm surges overtopped the concrete floodwalls near the lake, then undermined the earthen levees on which they were perched, setting the stage for the breaches that emptied the lake into the city.

Johnston said the Corps intends to launch an investigation to make sure it is correct about that scenario. But he emphasized that Katrina was a Category 4 hurricane when it smashed into the Gulf Coast, whereas Congress authorized the Corps to protect New Orleans against a storm only up to Category 3. “The event exceeded the design,” Johnston said.

The latest preliminary data would seem to punch a rather significant hole, as it were, in Johnston’s theory. But it was a crappy theory to begin with. Even if Katrina was a Category 4 at landfall, it was AT MOST a Category 3 by the time it drew parallel with New Orleans — and it didn’t make a direct hit on New Orleans, so the city itself experienced AT MOST Category 2, and more likely Category 1 or even strong tropical storm-force sustained winds.

Bah.

The article quoting Johnston was published on September 20, as Rita was threatening the coast, so it got overshadowed. I heard about it while waiting at O’Hare for my flight to Connecticut. So, that’s why I didn’t blog about it sooner. Sorry about that.

Regarding the new data on Katrina’s strength, The Storm Track adds: “This has far reaching implications for our understanding of storm surge. We know that the Gulf Coast saw huge storm surge, especially in Mississippi, and would not expect that size surge out of a Category 3 storm.” Indeed; Katrina produced higher storm surge than the Category 5 monster Camille. How is that possible? I hope we’ll eventually figure out the answer to that question.

Also confusing: wasn’t Katrina’s minimum central barometric pressure extremely low, even for a Category 4 storm? Wasn’t its pressure actually lower than Andrew’s at landfall? If so, how can that possibly jive with the idea of it being a mere Category 3?

Clearly, we have a lot to learn yet — about Katrina, and about hurricanes generally.