It's been 3 decades since Wisconsin had an EF-5 tornado. Are we 'due' for another?
(WLUK) -- Nearly 30 years ago, the last EF-5 tornado to strike Wisconsin ripped through the small town of Oakfield in Fond du Lac County on July 18, 1996.
And nearly 12 years ago, the last EF-5 to strike anywhere around the world decimated Moore, Oklahoma on May 20, 2013.
But since then? Not a single tornado has been given that highest EF-5 rating by the National Weather Service.
While EF-5 tornadoes account for less than one-tenth of a percent of all tornadoes, the historical odds of seeing one in any given year are still over 40%. The odds of going just ten years without one are less than a half a percent -- let alone our current streak of 12 years.
Wisconsin's historical rate of scale-topping twisters is much lower, with one only expected every 22 years or so. But it's now been 29 years since Oakfield.
So what gives? Are we seeing fewer tornadoes? Are tornadoes getting weaker?
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Dr. Tony Lyza is a researcher with NOAA and the National Severe Storms Laboratory. He co-authored a paper released this year titled "Where Have the EF5's Gone? A Closer Look at the 'Drought' of the Most Violent Tornadoes in the United States."
"At some point, the odds of that random chance being completely random and not being linked to something else, such as a change in rating practice, starts to decrease. And that's why we looked at the statistical analysis we did in the paper," says Dr. Lyza.
He says it likely has to do with how we rate tornadoes since changing from the Fujita, or F-scale, to the Enhanced Fujita, or EF scale, in 2007, rather than the tornadoes themselves.
The EF scale is based on newer understandings about what kind of damage can be caused by what kind of winds. But when developing the scale, some of the wind speed cutoffs ended up at very odd numbers, which were rounded off.
And in the case of the cutoff between EF-4 and EF-5 damage, that cutoff ended up being a significant one.
"EF-4 ending at 199 had to be adjusted to EF ending at 200, which pushed the single family home from being swept away from starting as an example of EF-5 damage to starting as an example of EF-4 damage," says Dr. Lyza.
At the time, one mile per hour may not have seemed like much of a change. But the "single family home" Dr. Lyza mentions is typically the only kind of structure impacted by tornadoes that doesn't get destroyed by much lower winds.
In other words, it's usually the only thing left for storm surveys to use when splitting hairs between EF-4 and EF-5 ratings.
His research indicates that this rounding may have unintentionally led to lower numbers of EF-5 tornadoes -- rather than this EF-5 drought being an indication that the strongest tornadoes today aren't as strong as tornadoes 10 years ago.
This doesn't mean the EF scale is broken, but it might mean it needs some tweaks.
"The meteorological and engineering communities are aware of it, and there's been some discussion about potentially addressing, but no definitive action has been taken yet," according to Dr. Lyza.
As for what this means for Wisconsin, just because there hasn't been an EF-5 since 1996, or even an EF-4 since 1994, that doesn't mean we're "due" for one.
Violent tornadoes aren't like the winners in a stack of pull tabs where you know that one has to eventually hit. Each tornado event is statistically independent of the last.
So, while going so long without an EF-5 rating might be odd, it's more a matter of human recordkeeping than an indication of nature's most powerful phenomenon getting any less dangerous.
Every tornado is capable of being deadly, regardless of what we rate it.
