The tornado on the night of Friday, December 10, 2021, that devastated parts of Arkansas, Tennessee, Missouri and Kentucky was part of an unseasonal outbreak, fueled by very mild surface temperatures. The peak season for tornadoes in this area is usually in March and April. If confirmed to be indeed a single tornado track of over 250 miles (400km) with a lifespan of over four hours, it would set a new record for tornado distance and duration. Beyond this single tornado, further twisters affected other towns in Kentucky, Kansas, and Illinois, including an Amazon warehouse. While casualties are still being recovered and damage assessed, it is likely the deadliest and costliest tornado outbreak in decades.
Quad-state tornado: a new devastating record?
Bastian, can you tell us more about what made this tornado so destructive?
Did climate change play a role in this unusual tornado outbreak?
Tornado outbreaks are the type of extreme event that scientists have the least confidence in attributing to climate change, unlike heatwaves or floods. This is in part because of the short-lived nature and limited geographical extent of these events but also because of the complex interplay of meteorological conditions that are required for tornadoes to form.
Early December 2021 saw some of the mildest weather ever experienced in the affected southern states, breaking multiple local temperature records for that time of the year. These are linked to the exceptionally warm Gulf of Mexico and the on-going La Niña phenomenon, which ultimately stems from a temperature imbalance of the Pacific Ocean but has global seasonal weather impacts. The combination of warm, moist air near the surface combined with cold, dry air at higher altitudes due to the onset of winter in the United States are the perfect mix for potent thunderstorms. As the moisture rises the thunderstorms swell to so-called supercells, like the one travelling from Arkansas across Kentucky, which spawn and fuel tornadoes.
Thus, a warming climate will likely lead to atmospheric conditions favorable of winter tornadoes as was observed over the weekend. However, other factors such as decreasing wind shear in a warming world could inhibit tornado formation. To date climate models cannot capture the small-scale meteorological dynamics that control tornadoes, making future projections highly uncertain.
Bastian Manz Senior Atmospheric Risk and Climate Risk Analyst at Allianz Re
How has the damage caused by tornadoes evolved over time?
To date, there have been no definitive trends in terms of tornado numbers across the United States in recent decades. However, increased losses can be attributed to two drivers. On the one hand, there has been some evidence that while the total number of strong tornadoes has remained stable, tornadoes are clustered over fewer days, so the damage of individual events becomes more widespread. On the other hand, the concentration of human population and assets continues to increase, which drives losses if these areas are impacted by a tornado, even if tornado strength and frequency were to remain unchanged.
What are the implications for the insurance industry?
In terms of quantifying losses, accurate location information and the use of geo information systems (GIS) is crucial, as tornados completely destroy structures in their paths while those close by remain unaffected. Insurers will look closely at the Quad-state tornado to better understand the damage dynamics of this specific event. This is particularly relevant if tornadoes continue to move from the traditional U.S. Tornado Alley into the southeastern states that generally have weaker constructed buildings and homes. Tornadoes are also a relevant peril outside the United States, as the extensive damage in the Czech Republic showed this summer.
Insured losses from so-called secondary perils have been increasing globally over the last two decades and exceeded those of traditional primary perils (wind, earthquake) in 2020. Amongst these secondary perils, tornadoes, straight-line winds (“derechos”) and hail (collectively termed severe convective storms) constitute the biggest loss drivers worldwide, despite their localized geographical footprint.
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** As of December 31, 2023.
** As of December 31, 2023.
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Allianz SE Reinsurance
Allianz SE Reinsurance
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