What causes tornadoes?

What causes storm systems to spawn dozens of tornadoes over the course of a few days?

• In the last few days in Oklahoma there have been dozens of tornadoes including the deadliest since 1947. What causes these storms to all happen within a few days of each other?

• Answer:

  Storms are the mechanism that re-hydrates (moisturizes) the upper atmosphere.  When the upper atmosphere becomes generally drier the energy (specifically, the high winds harbored in the jet stream) in the upper atmosphere cannot be contained and trends down in the atmosphere.  If it finds a pathway to large amounts of moist air in the lower atmosphere large amounts of energy can be directed from jet stream to ground.  But there is a lot more to it than just this: http://www.solvingtornadoes.com