Why Twisters Hammer Tornado Alley

DESTRUCTIVE PATH: A tornado that touched down in Moore, Oklahoma in 2003 tore a path of destruction through town, but paled in comparison to a vortex with winds above 320 kilometers-per-hour on May 20 of this year. Image: Courtesy of NASA Space Imaging

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The tornado that devastated Moore, Okla. has now officially been placed into the highest category?a five on the so-called "Enhanced Fujita" scale, meaning winds in excess of 320 kilometers-per-hour. Scientific American corresponded with meteorologist Harold Brooks, a senior scientist at the National Oceanic and Atmospheric Adminstration's National Severe Storms Laboratory in Norman, Okla.?

[An edited transcript of the interview follows.]

What are the key ingredients of a powerful twister like this? What conditions make it possible? Were the warning signs there for such a tornado in the previous days weather?

Low-level warm, moist air with dry, relatively cold air aloft, strong wind shear (change of the horizontal winds in the environment with height). The Storm Prediction had been forecasting the probability of a severe thunderstorm outbreak since Wednesday, 15 May, and mentioned the possibility of strong tornadoes by Friday. On Monday morning, looking at the output of high-resolution numerical weather prediction models that can resolve individual thunderstorms, the local forecast office mentioned in an 11 AM online briefing that, if a thunderstorm formed in the [Oklahoma City] metropolitan area that afternoon, it was likely it would become severe and produce a strong tornado rapidly.?

What can be done to protect oneself against a twister of this magnitude?

Get as low as you can and put as many walls between you and the tornado as possible. Get into a well-built structure. People survive tornadoes of this magnitude all the time. It may not be pleasant and they may get injured, but doing the right thing dramatically increases your survival chances.

Is there anything that can be done to better design structures to withstand such tornadoes? Are there types of buildings that are more resistant to such winds?

Mansard roofs are better than gables. Put hurricane clips attaching roof to walls and use straps and bolts with nuts to attach the wall to the sill plate.

How is it possible that multiple tornadoes follow very similar paths, as has now happened in Moore in 1999, 2003 and 2013?

The direction is largely a function of the wind profile in the atmosphere, so most tornadoes go from southwest to northeast. Other than that, the location is a matter of luck, most likely. If you threw a large number of darts at a wall, you could go up afterwards and draw a bullseye around a cluster. The process is essentially random, but even random processes produce clusters.

How much warning did the residents have and how did that warning go out?

The warning from the National Weather Service was 5 minutes before any tornado, 16 before significant damage and ~35 minutes before it got to Moore. The warning goes out on NOAA Weather Radio and through crawls on TVs and interruptions on regular radio. In addition, I assume outdoor warning sirens were sounded.

What can be done to improve such tornado prediction and forewarning?

Improve our understanding of all of the physical processes involved in tornadoes and the modelling of unresolved processes in numerical weather prediction models. From a user perspective, we need to do a better job of learning how to communicate information effectively.

What are the most dangerous misconceptions people have about tornadoes, or the common mistakes people make when faced with a potential tornado?

That you can't survive a violent tornado unless you're below ground. Some people probably still open windows, which is a bad idea.

Why do so many tornadoes occur in the U.S. compared to globally?

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