The US has borne the brunt of two spates of tornadoes in the past month. Between 14 and 16 April, storms swept through several states, with North Carolina the hardest hit. In the past few days a new wave has struck. Alabama's governor Robert Bentley said today that 131 people were confirmed dead in the state, and the number is expected to rise. New Scientist explains what is happening.
How many tornadoes have there been?
There have probably been 600 in April, according to Harold Brooks of the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma. That is more than any month since records began in 1950.
What is causing the onslaught?
For a tornado to form, cold air must sit above warm, moist air, and the wind needs to go in different directions at different heights, creating shear forces. "One thing that brings those things together is a strong jet stream," says Brooks, "especially if it comes from the west or south-west, over the Rockies."
That is exactly what has been happening throughout April. The jet stream has dipped south over the south-western US, sending strings of storms over the southern states for weeks on end. According to Brooks, such a sustained pattern happens only two or three times a century.
Is it over?
NSSL's forecasters think the stormy weather is likely to calm down over the next few days as the warm, moist air currently trapped over the southern states moves away and conditions become less favourable for tornadoes. But the laboratory is unsure what will happen next week: its different models are forecasting different outcomes.
Is climate change to blame?
There's no way of knowing for sure. There have been claims that tornado numbers have increased over the past few decades, but because records are less reliable further back in time, we cannot be sure.
"Some ingredients that are favourable to tornadoes will increase in a warming world, others will decrease," Brooks says. How that will balance out is anyone's guess.