Here in Tornado Alley, a swath spanning from Texas, Oklahoma, Kansas and Nebraska to Colorado, North Dakota, South Dakota and Illinois, the time to watch the skies extends from April through June, through the push-pull of Spring, when the fading fury of winter relents to surging swells of summer warmth.
This year, Tornado Season arrived early, and in places as removed from its provenance as Arkansas, Mississippi, Alabama and Virginia. Portions of the Midwest also were hit. In the first week of March, when much of the region typically lies blanketed in snow, a pair of EF-4 tornadoes ripped through sections of Indiana and Ohio, in addition to Kentucky and Tennessee. Later in the month, an EF-3 struck southeastern Michigan.
In addition to billions of dollars of damage, the outbreaks claimed dozens of lives.
The early arrival of spring may have had something to do with it. In Chicago, trees were flowering by mid-month. However, a new study entitled Tornado and Hail Risk Beyond Tornado Alley, by Santa Ana, Calif.-based analytics firm CoreLogic, suggests that more than the timing is off.
“The apparent increase in the number of incidents and shift in geographic distribution of losses that occurred last year in the U.S. called the long held notion of risk concentration in Tornado Alley into question, and is leading to changes in risk management policy and procedure,” said Dr. Howard Botts, vice president and director of database development with CoreLogic.
Time will tell whether changes in risk management prompt changes in codes in some jurisdictions. In the meantime, tornadoes continue to pose special challenges to designers and builders, according to John van de Lindt, a professor with Tuscaloosa-based Alabama University's department of civil, construction and environmental engineering.
“Though they address hurricanes and earthquakes, major code groups don't address tornadoes,” said van de Lindt, who spoke with ENR in the aftermath of a EF-5 tornado that last May cut a mile-wide swath across a six-mile stretch of Joplin, Mo., killing 150 people. “The problem is that tornadoes are unique because they not only subject buildings to the twisting loads associated with earthquakes, but uplift loads as well.”
Here's another problem: No one could have foreseen an EF-5 tornado touching down in Joplin, or anywhere else. E-5s are a rarity.
Rarer is the rule of thumb that accounts for the flowering of Chicago in early March, or the deadly tornadoes that ensued so early in that early spring.