In December 2021, a massive storm swept across the US, carrying heavy snow and rain into the West and northern Midwest of the country. In the South, which was enjoying near record-breaking heat via warm air from the Gulf of Mexico, the cold, dense air hit the warm air to create the perfect conditions for tornadoes. 

Just outside Searcy, a small city in Arkansas (pop. 23,660), the supercell, a strong thunderstorm with rotating updrafts, began spawning a family of tornadoes with astounding staying power. One ripped through the rich surrounding farmlands, destroying barns, hurling cotton bales with fury and destroying power lines. 

Another sliced through a nursing home in Monette and jumped the Mississippi River, where it plowed through the western edge of Tennessee. One EF4 tornado carved a 166-mile (267km) path of destruction. 

Altogether, 66 tornadoes were reported from Dec. 10-11, causing widespread power outages, damage and 93 fatalities. The devastating swarm of twisters marked the highest number of tornadoes recorded in December. This record stood only until Dec. 15, when a larger outbreak produced around 100 tornadoes across eight states. 

An American style of disaster 

All the famous cultural images of tornadoes — from Dorothy’s house being whisked away in “The Wizard of Oz” (1939) to the storm chasers in the surprise hit “Twister” (1996) – are set in the US. Why?  Tornadoes are predominantly an American phenomenon. 

The reason is the Great Plains, the broad expanse of flatlands stretching east of the Rocky Mountains and west of the Mississippi River. This vast interior of grasslands and prairie – including the famed Tornado Alley – is an ideal breeding ground for tornadoes. There, warm, humid air from the Gulf of Mexico meets cold, dry air from Canada and the Rocky Mountains. 

The dry air acts as a convection cap that prevents warm air from rising. The pressure builds until a cold front moves in and weakens the cap. Then the warm, humid air can burst out, billowing upward and swelling into 50,000-foot-tall thunderstorms in minutes. Some storms begin rotating through most of their depth and generate tornadoes. 

No other place on earth has the intense conditions of warm, moist air on the equatorial side and a wide, high range of mountains running north to south on the west side. This favors the mixing of different-temperature air masses, whereas in Europe, air convergence is hindered by the Alps running west to east. 

Is the tornado outbreak linked to climate change? 

The record-warm December helped fuel the fourth-warmest year for the US in 127 years of records. The temperature averaged 2.5F degrees above normal and fits into a long-term trend toward rising temperatures. All six of the warmest years on record in the US have occurred since 2012, says the US National Oceanic and Atmospheric Administration (NOAA). 

Yet, scientists have low confidence in detecting a link between tornado activity and climate change, primarily because of the short length of high-quality data records. However, some scientists say warmer winter temperatures will create conditions favorable to the formation of severe thunderstorms and tornadoes, but such effects are not yet detectable. 

NOAA noted the two December tornado outbreaks helped make 2021 the third costliest year of disasters on records. The $145bn price tag, of which some $85bn was insured, includes exceptional heat and drought in the western states, wildfires, a cold snap in Texas, four tropical storms and hurricanes, and nine severe thunderstorm/tornado events. The only costlier years were 2005, when Hurricane Katrina struck, and 2017, when hurricanes Harvey, Irma, and Maria made landfall. 

Secondary perils: a growing threat 

Insured losses from secondary perils have increased globally over the last two decades. Winter Storm Uri and other secondary peril events caused more than half of total insured losses in 2021. In 2020, 71% of all natural catastrophe losses resulted from secondary perils. 

Secondary perils include tornadoes, straight-line winds (derechos), and hail (collectively termed severe convective storms). Together, these constitute the biggest loss drivers worldwide, despite their localized geographical footprint. 

Derecho comes from the Spanish adjective for “straight” (or “direct”), in contrast with a tornado, which is a “twisted” wind. According to the US National Weather Service, a derecho is a band of storms where the wind damage extends more than 240 miles (400km) and includes gusts of at least 58mph (93kmh) along most of its length. Derechos can be hazardous to aviation due to wind systems such as microbursts, downbursts, and downburst clusters. 

Steps to avoid the worst if the storm hits 

To minimize losses in the event of a windstorm, businesses need to develop and implement a comprehensive written emergency plan. This should include actions to take before, during and after the storm arrives. 

The plan should cover areas such as training, assembling emergency supplies, business continuity, building inspections, anchoring, or relocating equipment and stock, protecting windows, flood protection, monitoring, salvage and recovery, and damage assessment. By implementing the appropriate preparations before a tornado strikes, a business can mitigate some of the damage caused by these devastating natural disasters. 

ABOUT THE AUTHOR

Andrew Higgins

Andrew Higgins is a senior regional technical and expertise manager for Allianz Risk Consulting at Allianz Global Corporate & Specialty.

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