If Yellowstone blows, this country is screwed. And then there’s the abrupt temperature change that will kill the food supply.
Worse. If we have another Yellowstone eruption the size of what happened 600,000 years ago, imagine effectively no sunlight for 8-9 months. And there enough pollutants in the air to cause substantial damage to the ozone layer. We're talking a near-extinction level event on a planetary scale.