Every year, an average of 1,200 tornadoes are reported in the US. While they can strike any time and any place, the most regular and violent activity strikes the central and southern states in the period roughly between March and June: tornado season.
Yet in the first months of 2024, there has been a strange and devastating uptick in twister occurrences.
April witnessed more than 100 tornadoes in the US in just one week, and, according to the National Centers for Environmental Information, a total of 384 were reported throughout the month. That is more than double the year-on-year average and the second-highest on record.
More than just numbers, the storms destroyed dozens of homes in Oklahoma, Nebraska and Iowa. An especially violent storm in the latter left four people dead and thousands without power, after severe weather flattened buildings in the town of Sulphur.
undefinedFootage coming in of Tornado damage in Sulphur Oklahoma#breaking #tornado pic.twitter.com/TkZXaF4WX9
— Dr.Duet🇵🇸 (@Drduet56) April 28, 2024
The past few weeks have also seen tornadoes wreaking havoc in unexpected places. Tornadoes in Nebraska and Iowa are much further north and east in the country than is typically expected for April, when tornadoes are usually concentrated in the south. Meanwhile, Wisconsin recorded its first-ever February tornado as higher-than-usual temperatures hit the state.
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What’s causing the dangerous change in pace?
To understand this, it's crucial to get to grips with what actually forms. “Tornadoes are areas of rotating air connected between the ground and a cloud above it,” Prof Jana Houser, a tornado expert at The Ohio State University, tells BBC Science Focus. “They range in intensity from 100km/h to over 500km/h [around 60 to 300mph].”
Houser explains that the way tornadoes form is quite complicated but you essentially need two basic conditions:
- Warm moist air rising into colder air above.
- Winds moving at different speeds and directions as you move up in the atmosphere, known as vertical wind shear.
This creates a rotating storm and generates an upward motion. “That upward motion then takes the rotation that's happening at the ground and basically pulls it together and sucks it up, intensifying in the process and forming a tornado,” Houser says.
The consequence of the complex processes responsible for forming tornadoes is that predicting where they’ll crop up and why is, well, tricky.
While no one is completely certain, experts think that the recent change in tornado behaviour could be a consequence of the global transition out of a period of El Niño.
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El Niño is a warming of the surface waters in the eastern Pacific Ocean, occurring irregularly every two to seven years on average. This disrupts normal weather patterns around the world, bringing drought to some areas and floods to others.
It’s also associated with warmer global temperatures, which is why, in concurrence with human-caused climate change, we’ve seen record-smashing temperatures in the last year or so.
Research published in the journal Environmental Research Letters has shown that as El Niño decays, atmospheric ‘waves’ which promote vertical wind shear become wavier, which in turn produces conditions ripe for tornado formation.
But Houser points out that we still can’t definitively attribute the changes in tornado patterns to this shift from El Niño to normal atmospheric conditions, and that any link remains hypothetical.
“There’s not necessarily a link, but there’s a possible link. It’s tempting to link these events together but we need to be careful because it’s not necessarily scientifically robust,” she says.
Conversely, in periods of La Niña – the opposite of El Niño, when average sea surface temperatures in the equatorial Pacific are cooler than usual – Houser explains that we might expect more wintertime tornadoes concentrated in the southeastern US states like Florida, Alabama and Mississippi.
According to predictions by the National Weather Service in early May, La Niña could form as early as June or August 2024 with increasing likelihood in the following seasons. A transition from El Niño to neutral conditions is expected beforehand.
What does this mean for people living in tornado-prone areas?
When tornadoes hit, they wreak havoc on communities, leaving behind a trail of destruction. Predicting where they’ll crop and providing people with early warnings is critical to minimise their impacts.
Thankfully, forecasters are getting better at this. Weather experts are now able to predict when the conditions for tornado formation will be ideal, and as soon as one is sighted or picked up on radar a warning can be sent immediately.
According to the NOAA National Severe Storms Laboratory, we could someday have models capable of predicting where individual tornadoes will arise too, though this remains some way off.
While, as El Niño ends, we’re likely to see a return to a “business as usual kind of model”, says Houser, there is still a lot of variability year-to-year that makes building an effective seasonal outlook tricky.
More worrying, though, is changes that are occurring on longer time scales. As our planet warms in response to rising levels of greenhouse gases in the atmosphere, experts are anticipating tornado hotspots to geographically shift in coming years.
Historically, the area centring around states like Texas, Oklahoma, Kansas and Nebraska – known as ‘Tornado Alley’ – is where the most dramatic activity takes place. As these parts dry out due to climate change, however, larger numbers of twisters are forming in southeastern regions.
This raises a serious problem: unlike the largely unpopulated plains found in Tornado Alley, the potential new hotspots encroach on more densely populated areas with greater infrastructure that could be destroyed.
As well as this, tornado clustering – where more occur in a short space of time – might become more common. According to a 2019 study, while just 11 per cent of tornadoes between 1950 and 1970 occurred on days where there were 20 or more tornadoes, since 2000 that number has crept up to 29 per cent.
“We’re not necessarily expecting the number of tornadoes per year to change, but they’ll become more concentrated on particularly high impact days,” Houser says.
This only increases the need for effective forecasting, since people may need to find shelter from multiple twisters in quick succession. Usually, a tornado lasts for a matter of minutes, but if, say, tens of them come at once, those people trapped in their path will require refuge for days.
After a record-breaking start to tornado season in 2024, with any luck the effects of El Niño are wearing off and things will return to relative normality. As for the coming years, well, that’s anyone’s guess.
About our expert
Jana Houser is an associate professor in the Department of Geography at the Ohio State University. She specialises in radar analysis of tornadoes and the supercell thunderstorms that commonly produce them by using state-of-the-art mobile radar observations.
She is currently studying the interaction of tornadoes with the ground beneath, addressing the problem of how topography and land cover impact tornado intensity and path. Houser has authored or co-authored 19 peer-reviewed journal articles related to tornadoes and supercells, most of which have been published by the American Meteorological Society.
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