The tornadoes that ripped through six US states on Dec. 10 were some of the deadliest on record. As of Sunday Dec. 12, at least 90 people have died. The governor of Kentucky reported at least 80 deaths in that state, and several more deaths were reported in Illinois, Tennessee, and Arkansas. US president Joe Biden has declared a federal state of emergency for Kentucky.
Tornadoes are fairly common in the central US, which experiences roughly 1,000 of them every year, but they rarely cause so much destruction in a single event. In fact, the death toll associated with tornadoes has declined significantly since the 1970s, with a few exceptions.
While the specific intensity of any given storm is difficult to predict, Friday’s storm brought together a unique set of factors that contributed to its severity.
A rare December storm
The powerful thunderstorm system that gave rise to the tornadoes was the result of unseasonably warm, humid air combined with a powerful cold front created by the La Niña weather pattern that the US is currently experiencing.
But what set this storm apart was the distance over which it was able to stay strong. The storm traveled 250 miles across Arkansas, Missouri, Tennessee, and Kentucky over the course of roughly three hours.
While these conditions were unique, scientists so far say that it’s difficult to clearly attribute them to climate change. Individual tornadoes are short-lived, variable events that are more difficult to link to a warming climate compared to events like wildfires or heat waves.
There is more consensus, however, around the fact that a warming climate will lead to more extreme winter storms. Victor Gensini, a meteorology professor at Northern Illinois University, told ABC News that these storms are “becoming more common because we have a lot warmer air masses in the cool season that can support these types of severe weather outbreaks.”
Tornado alley is on the move
The six states where the tornadoes hit are all part of the larger region vulnerable to tornadoes, but they aren’t in the epicenter of what’s traditionally been considered “tornado alley,” where these storms have historically been most frequent. There’s no official definition of tornado alley, but it’s generally considered to include the central plains and southern states of South Dakota, Iowa, Nebraska, Kansas, Arkansas, Oklahoma, Louisiana, and Texas.
However, scientific research suggests that tornado alley has been moving eastward. One 2018 study found that over the past 40 years, severe storms in the southeast US have become more frequent, especially in places like Mississippi, Missouri, Tennessee, and Kentucky. At the same time, the number of severe storms in Texas and Oklahoma has gone down. Many of the most destructive storms of the 21st century have happened east of the Mississippi river.
Friday’s storms corroborate this pattern; the worst impact was in eastern Arkansas and western Kentucky.
The southeast US is especially vulnerable
As tornado alley has shifted eastward, storms are striking more vulnerable communities. A Feb. 2021 study evaluating the relationship between tornado warnings issued by the National Weather Service and societal vulnerability found that places in the southeast were the most impacted by tornadoes because more frequent storms were touching down in places with existing vulnerabilities like high poverty rates. These areas also have more people living in manufactured homes, which are less structurally sound against the high winds of tornadoes. Since 1992, 42% of tornado-related deaths in the US were in manufactured homes.
In Kentucky, which was badly hit by this most recent storm, manufactured homes make up roughly 11% of the housing stock. Early reports from local news outlets say that at least one mobile home was flattened.
Storms at night are deadlier
Timing also plays a huge role in the lethality of tornadoes in general. On Dec. 10, the first tornado touched down in Arkansas at around 7pm, and successive sightings were reported throughout the night, the deadliest time for tornadoes to hit. A 2010 study found that tornadoes that happened in the overnight hours caused nearly 2.5 times more casualties than tornadoes that hit in the early afternoon, and nearly 50% more injuries.
Researchers point to a few reasons for why this may be; in general, when people are at home and sleeping they may not be able to respond quickly to tornado warnings, but the types of homes people reside in have an impact too; manufactured mobile homes, without strong foundations or basements, are more vulnerable to the high winds from tornadoes, and the general guidance for people living in them is to seek shelter elsewhere when a tornado hits. At night that’s not always possible to do.
It’s too early to say what role timing played this time. The largest single fatality event came from a Kentucky candle factory, where 110 people were working an overnight shift. The building completely collapsed when the tornado hit, and only 40 people were rescued, according to the latest reports.