Wednesday, April 18, 2007

What If A Tornado Hit A Heavily Populated Area?

The photograph is unforgettable. Captured by a businessman on his way back to the office after lunch, the picture clearly shows a tornado in downtown Miami. It was May 12, 1997, the day a twister dropped out of the sky and into the history books.

According to a National Weather Service report, the tornado developed just southwest of Miami touching down in the Silver Bluffs Estates a few minutes before 2 o’clock in the afternoon. Moving east northeastward at around 20 mph, the tornado crossed I-95 sweeping through downtown Miami before entering Biscayne Bay near Bicentennial Park. The tornado continued northeast crossing the MacArthur and Venetian causeways before weakening over Biscayne Island.

Described as “a significant F1” tornado, the weather service estimated the twister’s wind speed at 100 to 110 mph. Even though it cut a 150-yard path, eight miles long through the heart of downtown Miami, the storm only caused minor injuries (mostly cuts from flying debris) and no deaths. Damage was estimated at a little more than $500,000.

Thousands of people witnessed the event with many snapping photographs of the tornado as it danced among the city’s skyscrapers. The Miami tornado made newscasts and headlines around the world, marking the first time a twister had hit a major metropolitan area.

Still, the people of downtown Miami were lucky that day. The tornado was weak and only last a few minutes. What would happen if a much more powerful tornado had hit the city, a tornado with winds of 300 mph and a width of over one mile?

Sound crazy? Don’t tell that to Joshua Wurman and Paul Robinson, two scientists from the Center for Severe Weather Research in Boulder, Colorado. Wurman and Robinson are convinced that someday a large, intense and long-track tornado will impact a densely populated urban or suburban region.

That worst-case scenario almost happened in Oklahoma City, Oklahoma on May 3, 1999. More than 70 tornadoes touched down in the Central Plains that afternoon including the last F-5 twister recorded in the United States.

The National Weather Service reported that the F-5 tornado had winds of 300 mph as it tore through the Oklahoma City suburb of Moore. The powerful storm destroyed more than 10,000 buildings, produced $1.1 billion in damage and claimed 36 lives.

Wurman and Robinson wanted to know what a similar tornado would do if it hit a heavily populated urban area. Using computer models and data from past tornado outbreaks, the two scientists created a large, powerful simulated tornado hitting Chicago, Illinois.

The result of such an event would be catastrophic. Wurman and Robinson, writing in the Journal of the American Meteorological Society, say that such an event “could cause widespread damage and loss of life on a scale that has not been observed historically with tornadoes.”

Wurman and Robinson’s computer model suggests that a powerful tornado, like the one that hit Moore, would “destroy structures across more than 50 miles, killing perhaps 10% of the residents in these structures, resulting in as many as 13,000 to 45,000 deaths in densely populated cities like Chicago.”

The scientists recommend that emergency managers in highly populated areas have contingency plans for such an event. While admitting that a powerful tornado striking a major city has not occurred, Wurman and Robinson say such an event is inevitable. Through increased awareness, preparation and reliable weather warnings, the impact of a powerful tornado hitting an urban area may be reduced.

Posted at 9:24 AM