Tornadoes

Seeking Shelter on the Road
Weather.COM, Mar 3, 2000

You are travelling down a highway when the sky turns an eerie green, the wind picks up, and suddenly you are in the path of a ferocious twister. There's a highway overpass ahead - and a ditch to your right. Which do you choose? Seeking shelter under the overpass might seem like the best choice, but meteorologists warn it could be a deadly decision. "It's an extremely unsafe place, you are up off the ground where the winds speeds are very high," explained Dennis McCarthy, meteorologist in charge of the National Weather Service Forecast Office in Norman, Okla. At the Severe Weather Workshop in Norman, Okla., this week, tornado experts addressed the rising concerns over the safety of highway overpasses. Over the last two decades, public perception that these structures offered sound shelter has increased substantially, in part due to a well-publicized video of the 1991 Andover, Kan., tornado. The television news video showed people congregating beneath the overpass - and surviving. That's part of the problem, the video has been played and played and played, it's now embedded in the public's subconscious as the thing to do," said McCarthy. "But they were lucky. That tornado was very weak."

A Deadly Decision?
Concerns over the safety of highway overpasses reached a critical level on May 3, 1999 when tornadic supercell thunderstorms produced several long-tracked violent tornadoes that struck parts of central Oklahoma and southern Kansas. In fear for their lives, many people sought shelter from approaching tornadoes under highway overpasses. Twisters crossed interstate highways in seven different locations in central Oklahoma. At three of these locations, a highway overpass was in the direct path of the tornado. One person died at each of those three locations. "They basically put themselves at higher wind speeds," said John Scala, severe weather expert at The Weather Channel. "When the tornado went right over the location, the wind accelerated underneath the overpass, taking whatever was in its path." In addition to the three deaths, there were scores of severe, potentially life-threatening and gruesome injuries inflicted upon people underneath the overpasses that in some cases has left these people with permanent disabilities.

The tornadoes that struck two of the overpasses were exceptionally violent - F4 or even F5 intensity. "Like water through a small pipe" When a tornado strikes a highway overpass, the winds flowing through the narrow passage underneath can speed up. The increase in wind speed can strip the landscape, uprooting soil and debris and sweeping away everything in its path. "It's like taking water and running it through a small pipe versus a larger one," said Scala. "The water moves faster through the narrower pipe." This increase in wind speed is due to what's called the Venturi effect. Air is considered a fluid. When you force a fluid, such as air or water, down a tube with a constricted channel, the fluid moves faster through the narrow part and the pressure drops. An automobile carburetor relies on this principle. Get to the lowest level When a tornado threatens, meteorologists say the best advice is always to be prepared and get to the lowest level. On the road, that's a ditch in most places. "There's no one recommendation that will fit every situation. If the storm is literally on top of you and you had to choose between a ditch and an overpass, I'd still choose a ditch," Scala said.

In extreme cases, people have lost their lives in ditches, because debris and water can collect in the ravines. "The main thing is, there is so much information these days, people should get the information and avoid the situation all together, " urged McCarthy. In the case of the May 3, 1999 outbreak, McCarthy said the people were congregating under the overpass a long time before the tornado ever got there. "The safest thing for them to do was turn around and go away from the tornado. The information was there, the weather radio was there, it could have easily be avoided," McCarthy said. On Thursday, the American Meteorological Society issued its new policy statement on tornado preparedness. The AMS is underscoring the need for the public to follow basic safety rules and understand the fundamentals of the threat and danger of tornadoes. "These fundamental steps will save lives," said McCarthy. "The more a person understands about the storms, the geography, and the watches and warnings, and where to go for safety, the more likely they are to survive a tornado."