Maybe it was the prison at the point going up in flames
But really... I turned on the salt lake police frequencies, and didn't hear any chatter. So maybe nothing but a transformer? We Utahns know how
often the power goes out around here, and with the snow fall we just got, I wouldn't be surprised if that's what it was. I watched a transformer
blow near my house once, and didn't hear a boom... But it sure lit up the sky. I dunno.... All I know is I was driving home from a movie at 9:00.
Was in my kitchen (which is on the ridge and overlooks the valley) making coffee 9:15-9:20, and then in my garage having a cigar at 9:25-10:15. Sadly
missed what you saw...but think I would have seen it if it were huge, since the door to the garage was cracked.
Curious to know though.