I was at the Target on Union, and left about 3:00. I saw the black clouds of smoke pouring into the sky. I got onto eastbound SR16, and traffic was slow, because the flames were very visible and shooting into the sky; the fire department obviously didn't have control over it yet. People were parked on the Sprague overpass watching it, and as I approached the Valley overpass, some people had pulled off to the side. The flames were shooting up well above the north side of the overpass, not gently licking the sky, but jetting up as if they were being accelerated out.
Halfway across the overpass, I turned off the radio and opened the window, because I wanted to see if I could hear the fire. As I rolled down the window, the whole north valley burst into an explosive fireball. It was bigger than most Hollywood type explosions, rocking the car. Maybe the equivalent of four simultaneous gas station explosions, if they were done in a movie.
My first thought was that I was glad the window was down, so that the pressure inside and outside the car was equalized, preventing the window from shattering. My next thought, as the wave of heat swept over, was that I needed to roll up the window to shield myself from the heat. That was a bit too late -- the left side of my face is still a little pinker than my right, and it still stings a tad.
My next thought was, "gee, I'd better swerve left a little bit to avoid that falling, flaming piece of scaffolding.". I did, but it clipped the car in the lane to my right. KING 5 news interviewed someone else who saw flying metal, too.
I kept driving off the viaduct, adrenaline coursing through me. It had gotten warm enough that I wanted to check my car's paint to see if it had gotten damaged.
KOMO radio reports that the explosion was felt a few miles away. I believe it. I'd be surprised if the Java Jive, south of the overpass, is still in one piece.
[Update: The News Tribune just posted some good video of the explosion. I think that was my car that crossed the SR-16 supports about three seconds pre-boom. Toasty.]