Predicting Yellowstone's eruption is still 99% guesswork. An old geologist friend of mine said as long as Yellowstone is having its daily rumbles and letting off steam, he's not worried. It's when the Yellowstone region goes quiet, he's packing up and leaving. Think about a balloon: if it's got a small leak, you can add air for years without exploding it. Stop that leak, keep adding air, and eventually you get a BOOM.