It doesn't matter how advanced technology or warning systems are, there would still be people merely in the wrong place at the wrong time. Many of the victims claimed by Mt. St. Helens were either doing their job or were not in a restricted zone at all. They were simply victims of a cruel hand of fate. Other victims were in the wrong place by choice. That would not change either. Even if every individual was personally told that the mountain would erupt on or around May 18th, people would STILL have been there out of morbid curiosity. Human nature and the invincible attitude that comes with it has not evolved in 30 years time.
Would there always be the ornery old cuss who won't leave his home, come hell or high water? Yep. Would there always be the thrill-seekers and the looky-lous who show up with their coolers and cameras, ready to be entertained? Um, yeah! Even though natural selection takes out a few of those, that particular gene will never evolve and in this county, it only seems to mutate.
What I'm saying is that lives would be lost, no matter the decade. (Unless it's 3010 and everyone is made of robot parts, but that's another discussion altogether) Maybe less than 57 would have died on that day, but no amount of technology could prevent that. So what would have changed? Guess you'll have to check in on Monday to see.