I was a kid living in Kennewick when this happened. We were in church, and came out to see the low, heavy dust clouds over everything. Of course there were lots of jokes afterward about people thinking Judgement Day had come, but in actuality everybody knew it was going to blow for a long time preceding the actual eruption, so everybody understood instantly what the clouds were. Their three dimensional heaviness, however, was quite new and interesting.
This picture is great, and captures a little something of the blast.