consistent pattern of flashing light behind the clouds (on a clear night)

The sky is clear and the stars are out, yet behind the "background" clouds, there is a pattern of flashing lights as bright as lightning.  They can't be lightning because there is no crackle or spark of any kind, nor is there any following thunder.  The lights flash kind of like Morse code: flash-flash...flash...left flash...right flash...and then a streak of light.  Then the pattern repeats, which makes me assume it can't be a natural phenomenon.
My dad and brothers went riding toward where they figured there must be a tower emitting the lights, but the farther they drove, the more they realized that the lights were not being emitted from anywhere on the ground.  They're positioned in the middle of the sky and they're not moving, as the lights of an aircraft would as it flies.  The flashes are also way too bright and large to be aircraft-based.  My dad describes the streaks after the flashes as looking like "particle streaks".
They've been going on for a while now (about thirty minutes) and they show no sign of stopping.  Other people are coming out of their houses and driving to get a better look.