Pulsing green light over portland

I was driving east and in my peripheral saw a bright flash to the north. I looked in that direction thinking it was lightning, and I saw that it was light green in color.  It seemed to fade and pulse brighter.  My next thought was that maybe a vehicle had collided with an electrical transformer (It has been snowing, so there is ice),  but I heard no explosion. For about 10 seconds, it pulsed in brightness, until fading entirely.