No one knows exactly how much warning we’d get before an eruption at Yellowstone. There hasn’t been a supervolcano eruption in recorded human history. It’s possible it could happen suddenly, but more likely there would be years of earthquakes and topographical changes to warn us. The USGS maintains a volcano observatory at Yellowstone–you can see their forecast of the risk of an eruption here (updated monthly). So we’re likely to get at least some warning of a Yellowstone eruption. Whether we’d prepare adequately is another question, of course.

Even the well-forecast and relatively small (compared to Yellowstone) eruption at Mount St. Helens killed 57 people, in part because the eruption came in a direction (sideways), time, and magnitude that wasn’t possible to predict precisely. Hurricane Katrina killed 1,836 people, even though we’ve known basically forever that a Category 3 hurricane could hit New Orleans, and we had a three day warning before it did. So it’s entirely plausible that the fifteen-year-old protagonist of Ashfall might not believe the Yellowstone volcano posed any threat to him in central Iowa. It would be less believable if he were concerned about it in advance. There a detailed post about the science behind Ashfall on the Our Time in Juvie blog here.

The Puyehue-Cordón Caulle eruption on June 4, 2011 happened with fewer than 4 hours notice. There’s a fabulous set of pictures of the eruption’s aftermath here.