We didn't see a UFO or anything, this was an ear-splitting almost explosion-type burst that left the whole horizon a luminous semi-circle of purple. We had been driving through a snowstorm; it was still snowing so we didn't feel that it could have been lightning; we saw nothing like lightning, only heard a gigantic "wave" of sound similar to the thunder accompanied by lightning, then saw the entire horizon light up in a bright purple hue. My 3 children and I were just getting off of I-70 west heading to a Denny's at which the waitress admitted she had been witness to the same.