This year the storms in the US have been devastating, with at least 40 people killed in just three days.

After barreling through Oklahoma and Arkansas on Thursday, the system significantly worsened on Friday, producing an incredible 113 tornado reports.

Worst hit was North Carolina, which saw its deadliest day for tornadoes since 1984.

This area of the US regularly sees tornadoes at this time of year.

Called Tornado Alley, it's the region to the east of the Rockies, and between May and June passing weather systems commonly turn severe and spawn tornadoes.

The treacherous weather is a result of fighting weather systems the cold air flowing down from Canada smacking into the warm air that’s trying to bring spring up from the Gulf of Mexico.

The bigger the temperature difference, the more explosive the result: Immense thunderstorms with flooding rain, giant hail and if you're unlucky, the odd tornado as well.

This year the storms have been more severe than usual, and perhaps there is a reason for this.

La Nina is the name given to a subtle change in the Pacific Ocean, when the temperatures of the surface water are a little colder than usual. 

The drop is only about 1.5 degrees Celsius, but for the ocean, this is a large change, in fact the largest seen in decades.

And it's already having a significant effect on the weather around the globe.

La Nina has caused floods in Australia and Sri Lanka, as well as a drought in the Horn of Africa.

Another symptom of La Nina is to change the weather across North America.

Over the next few months, La Nina conditions are expected to bring below-average temperatures across the northern tier of the country and above-average temperatures around the Gulf of Mexico.

This would obviously increase the temperature difference across the US and would in turn lead to more intense storms, as we have already seen.
 
Certainly the severe weather isn't calming down yet and more tornadoes are expected in the coming few days.