SEATTLE - Tornadoes are very complex systems and thus are difficult to forecast. First, you need a strong thunderstorm and warm, moist air near the ground. Strong thunderstorms will have strong upward moving winds as the warm air races upward, and downward moving winds from the heavy rain and hail falling.
These wildly varying winds will cause winds to be flowing from different directions near the ground. The colliding winds will cause a spinning horizontal tube of air along the ground. This tube will then get picked up by an updraft and brought into the thunderstorm, now making the cone of spinning air vertical. It's this spinning combined with strong updrafts that make a tornado, and why tornadoes work like a vacuum -- they're pulling air upward.
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