He backed his car out of his garage as a tornado- a fierce storm, was approaching. When he turned around to go back the tornado was destroying his garage and only a bit of it was left.

back a car out of somewhere = to reverse (drive backwards) out of somewhere tornado = very strong wind circling from the sky to the ground, (imagine cows flying everywhere, such strong wind) http://en.m.wikipedia.org/wiki/Tornado So he was driving his car out of his garage, the tornado started, he decided to stay instead (probably safer), looked back and the garage was almost gone (blown away by the wind). Hope this helps