In this case, the extremely cold US (winter) was probably a contributing factor. Tornadoes in the US form when a front of dry, cold air descending from the north meets warm, moist air coming up from the south. Sometimes, a body of the cold air slides over the top of the warm air, trapping it underneath.Ohh, this is how.
Extreemly cold winter causes tornados, but it's because of global warming?
There is no evidence that the world's warming climate is leading to an increase in tornadoes, says Terence Meaden, deputy head of TORRO.
"Higher average temperatures mean the air is able to contain more water vapour, which increases the tendency towards more severe thunderstorms, and more severe thunderstorms tend to produce tornadoes," he says. "But are the small average temperature increases that we're seeing across the planet having any influence on tornadoes? I don't know."
New Sientist peice here