The number of tornadoes per year bounces around, but the average has been relatively stable since record accurate record keeping began over 100 years ago. But the number of powerful, deadly tornadoes and the number of multiple vortices popping up in clusters is on the rise, and meteorologists aren’t sure why:
When weather conditions are just right — lots of rising heat and moisture, and vertical wind shear — sometimes you get more than just a tornado. Mathematician Micheal Tippertt at Columbia University, who tracks these outbreaks, says that while the number of tornadoes nationwide varies a lot year to year, the overall average is pretty steady.
"But the number of tornadoes in outbreaks is increasing," he says. And the number of tornadoes in the most extreme outbreaks — those where at least a dozen tornadoes hit a region within one to three days — is increasing the fastest. Scientists who study climate suspect that warming temperatures may affect how many tornadoes we get. After all, warmer, wetter conditions are like priming the tornado pump.
On first glance, the obvious culprit would be global warming. But tornadoes are not hurricanes or heat waves; they are not heat engines in the narrow sense. Studying their origins is the purview of tornado genesis and what those researchers have found is that tornadoes are far more chaotic than typhoons and hurricanes. So by extension, tornado clusters may not be clearly linked to climate change as well.
The US sees more twisters than any other country, the vast majority of them occur in Tornado Alley stretching from Texas and Louisiana to the Great Lakes on both sides of the Mississippi River. One major tornado is bad enough, if these start roaming over the plains in packs it would be a nightmare come to life.