Are tornadoes and tornado outbreaks becoming more common?

Weather
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Peoria, IL – While the answer should be simple, people often spin tornado stats to fit a certain narrative. I know…big surprise. The goal of this article is to provide some context to some of the many stats you may see floating around the internet when it comes to tornado trends. 

Are tornadoes and tornado outbreaks becoming more common?

The short answer…

  • There is a slight upward trend in the number of tornadoes that have touched down in Illinois since 1950.
  • The amount of tornadoes, especially strong to violent tornadoes, has seen a steady downward trend nationally.
    • No EF-4 or EF-5 tornadoes hit the U.S. in 2018.
  • The number of large tornado outbreaks and the number of tornadoes in those outbreaks are increasing despite lower tornado counts.​​​​​​​
  • None of these trends are linked to climate change​​​​​​​

Illinois Tornado Trends

A quick look at the stats from the State Climate Office of Illinois shows a distinct positive trend in the number of tornadoes (EF-0 to EF-5) to touchdown in Illinois. While this trend seems alarming and could easily be misconstrued as an impact from climate change, it’s important to understand the methods of data collection and how those methods have changed over the years.

EF-0 tornadoes rarely cause any damage and were largely ignored unless they impacted a populated area, that is until the last 10-20 years. Recent improvements in technology, population growth, improved spotter networks, and increased public awareness have all contributed to more of these weaker tornadoes being reported.

To get a more accurate representation of tornado trends its best to look at the stronger tornadoes (EF-1 to EF-5). You’ll see that while the trend is still positive, it’s much weaker than the trend that includes EF-0 tornadoes. Strong tornadoes are more likely to cause moderate damage and would have been well documented, even in the past. While the past reports are still likely underrepresented due to smaller urban areas in the 1950s and 60s, it’s a closer representation of what the trend actually looks like.

A recent study by NIU Professor Victor Gensini, Ph.D. and Harold Brooks of the National Severe Storms Laboratory found that increases in tornadic activity in areas like Illinois could be the result of an eastward shift in “Tornado Zones”, or an area known as Tornado Ally. This study, which I wrote about in 2018, used the Significant Tornado Parameter (STP) index to identify where tornadic activity has increased and where it has decreased. It’s important to note, that while the study shows there is a link between STP and tornado reports, its not an exact representation of tornado trends.

National Tornado Trends

Data from NOAA and the Storm Prediction Center show no real discernable trend in the number of tornadoes of EF-1 or stronger between 1954 and 2014. If anything, the trend shows a very subtle decrease in tornadic activity.

You can see this negative trend a little clearer when you look at the number of strong to violent tornadoes (EF-3 or stronger) to touch down over the same time frame. This trend of few tornadoes is quite astonishing when you consider that if tornadoes occurred at a constant rate, tornado reports should generally increase with time with improvements in technology and population growth. Instead, tornadic activity has been decreasing.

Ian Livingston of the Washington Post highlighted this trend in an article he wrote on December 26th, 2018. Using data from the Storm Prediction Center, Livingston shows a steady decrease in the number of violent tornadoes (EF-4 and EF-5) to touchdown in the United States. Interestingly enough, 2018 was the only year since 1950 where an EF-4 or EF-5 tornado did not touch down in the U.S. (The only violent tornado to occur in 2018 globally occurred on August 3rd near Alonsa, Manitoba in Canada). 

Tornado Outbreaks

A recent chart put out by Climate Central states that the number of tornadoes in large tornado outbreaks is on the rise. The data comes from a study published in the journal Science in 2016 titled “More tornadoes in the most extreme U.S. tornado outbreaks” Tippett et al (2016). The data in this study certainly shows an increase in the number of tornadoes during the largest outbreaks since the 1950s.

While I don’t dispute the facts of the study above, I do question their definition of a tornado outbreak. In this study, they define a tornado outbreak as six or more tornadoes in six consecutive hours nationwide.

In the past, the definition of a tornado outbreak has been relatively fluid, generally ranging from 6 to 10 depending on the study. According to the American Meteorological Society, a tornado outbreak is 10 or more tornadoes from a particular synoptic-scale system.  The stricter guidelines of what constitutes a tornado outbreak from this definition would then lower the amount reported outbreaks in the previous study by Tippett et al (2016).

As an example, let’s say that one storm system over the southeast has produced 4 tornadoes in Alabama and another system over the Central Rockies has produced 3 tornadoes over Colorado, all within a six-hour period.

According to the definition laid out by Tippet et al (2016), this event would classify as a tornado outbreak, even though two separate systems, in two separate geographical areas, produced the 7 reported tornadoes. However, if we were to use the definition from the AMS, this particular event would not classify as a tornado outbreak as it didn’t reach the 10 tornado threshold and the 7 that did were part of two separate systems.

While it seems like a moot point, I think it’s important to have a more complete and firm definition of what constitutes as a tornado outbreak for future studies on the topic.

Are these trends linked to climate change?

While many have theorized that climate change would lead to an increase in tornadic activity, the trends don’t reflect it.

The oversimplified version of whether or not climate change would lead to more tornadoes often centers around the theory that a warmer climate leads to more instability, which in turn would lead to more and stronger thunderstorms. However, the real driving force in tornado development is wind shear and it’s not yet clear if and how climate change could impact it.

It’s also worth noting that the tornado data we have today is a small sample size, especially for trying to detect long term trends. Thanks to vast improvements in data collection and public awareness, long-range trends, if they exist, will eventually become clearer with time.  

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