The outbreak of ferocious tornados that struck the US on Friday night was horrific. Even though they had been forecast a day in advance, they struck at night when most people were asleep and unable to take shelter. Worst hit was Kentucky where possibly 100 people died, but the final number of casualties may remain uncertain for some time.
One exceptionally powerful tornado carved a path of destruction extending more than 200 miles over more than four hours. It was possibly the largest yet recorded on radar by the US National Weather Service. Radar also revealed that debris from the tornado shot up as high as 37,000ft. Possibly the closest comparison was a monster tornado on March 18, 1925, which carved a path of 219 miles through four states and killed 695 people.
The recent tornados were also shocking because they struck in winter whereas the typical tornado season is in spring and summer. The outbreaks on Friday were partly because this December has been very mild across much of the US, with temperatures 15-20C above average in places and new record high temperatures for December set in many locations.
Friday’s storms were fuelled by warm, humid air streaming up from the Gulf of Mexico where sea temperatures are unusually high. Added to that, winds higher up in the atmosphere were sweeping across in different directions and at different speeds, adding spin to the storm rather like a spinning top and helping to set the tornados off.
The engine driving all this strange weather has come from a bout of La Niña that is growing in the Pacific, and which is turning the equatorial tropical seas cooler than normal towards the Americas, but warmer towards Australia and Indonesia. Typically, La Niña makes winters colder and wetter in the northern US and Canada, but drier and warmer in the southern US as the cooler equatorial seas in the Pacific shunt the position of the jet stream northwards.
