Severe Weather Outbreaks Are Spawning More Tornadoes

Late spring is the peak of the severe weather season. Case in point are the twisters that spun up on the Southern Plains to the Ohio Valley earlier this week. The preliminary count includes 26 tornadoes reported on Monday, and 34 tornadoes reported on Tuesday. 
 

Some of the most striking damage comes on outbreak days, or days with multiple tornadoes within a geographical area. Outbreaks are among the most visible reminders of a twister’s power. The worst outbreak on record in the U.S.  occurred on April 27, 2011. That day, 199 tornadoes caused 216 fatalities, more than 2,400 injuries and resulted in more than $4.2 billion in damage. 

While extreme outbreaks like that are exceedingly rare, there are signs that they’re becoming more common.

The impact of climate change on tornadoes is a very active topic of research. The best way to understand how a warming world will affect tornado development is to examine how it will affect the environment favorable to produce them. The two main ingredients for that environment are energy and spin.

Research suggests that global warming could provide more energy, but less of the wind shear necessary to generate tornadoes. The energy, known as convective available potential energy (CAPE), is based on heat and humidity, and both of those increase in a warming world. Wind shear is the changing speed and direction of wind through the depth of the storm. A world that is warmer overall may have weaker jet stream winds and thus, less shear. This yields mixed results about what to expect regarding tornado development in the future.  

A Stanford study used several regional climate models to suggest that decrease in shear will be concentrated in areas and times when the CAPE is low. As a result, there may not be much decrease in the occurrence of conditions favorable for tornadoes.

Observations over the past few decades have yielded an interesting trend. While there are fewer days with tornadoes, there are more tornadoes on those days. Because there have been changes in how tornadoes are reported and detected, researchers eliminated the weakest tornado classification on the Enhanced Fujita scale known as EF0 tornadoes (or F0 before 2007 on the original Fujita scale).

Since the early 1970s, the average annual number of days with at least one EF1 or stronger tornado has dropped from 150 to 100. Yet, there has been an increase in the number of days with a very high number of tornadoes. In the 1970s, the average number of days with more than 30 EF1 or stronger tornadoes was a fraction less than one, meaning they weren’t even a yearly occurrence on average. In the last decade, that number had jumped to three days each year.

While there are fewer days each year with tornadoes, on average, the number of days with more than 30 tornadoes is increasing.

Credit: Climate Central

The states that make up the core of Tornado Alley — Oklahoma, Texas, Nebraska and Kansas — have all seen the number of days with one or more EF1+ tornadoes drop more than 50 percent in the last 50 years. But the increase in the number days with more than 30 EF1+ tornadoes is not as substantial there.

A more recent Florida State study supports the previous research indicating the risk of tornado outbreaks is increasing. But the specific reason for this increase in days with outbreaks is not certain. The Florida State study suggests that the upward trend may be related to a higher concentration of tornadoes rather than larger areas becoming more favorable for tornado development.

Given the damage they cause, the effect of a warming world on tornado frequency and intensity will continue to be an active area of research in the years to come.