Mitch Dobrowner has a vivid imagination. When he listens to music, he can see colors. And before he goes out to photograph, his mind's eye already sees an image of what he wants.

This time, it was something like a spaceship—the type of storm cloud that takes on a distinctive saucerlike shape.

His expectations were met—and then some—the first time he found himself facing a supercell storm on America's northern plains. He and Roger Hill, a veteran storm chaser who's been his guide ever since, had been following this particular storm for nine hours as it wound its way from Sturgess, South Dakota, through the Badlands, and down into Valentine, Nebraska, gathering force as it went.

"I couldn't believe what I was looking