OK, you're asking a seismologist about a volcano and if any of my vulcanologist buddies read this I'll never hear the end of it...

Let's clarify some obscure but important points. There are different kinds of volcanic eruptions, even at an individual volcanic center. At Yellowstone the most common kind of eruption is hydrothermal --superheated steam or water blasts out craters as large as half a mile across! But that isn't going to strongly affect the US. Magmatic (i.e. lava--molten rock -- the real stuff!) eruptions vary in the chemistry and physical properties of the magma they emit from soupy, runny, basalt like a Hawaiian volcano (picture lava flows running along the ground) to sticky chunky dry goobery gunk called rhyolite that can be as explosive as all get out. The commonest (i.e. most likely) eruptions are smaller ones, and caldera-forming super volcano explosions being the rarest. In fact. at Yellowstone the real bad boy explosions that frame the main point of your question occurred 2.1 million years ago and 1.3 million years ago and 0.64 million years ago. Based on average recurrence of these events, we are deep into a cycle and may only have to wait another 90,000 years for another. [I'm no statistician, but in order to actually answer your last question I just calculated a rough back-of-the envelope estimate of ~0.02% chance of a supervolcano eruption in 100 years, and I made some assuptions that make this rather a high-end estimate].

The most likely eruption, either hydrothermal or magmatic, would NOT be a super volcano, but a regular-sized family-friendly volcanic episode. It would be big news. It would close the Yellowstone National Park. It would likely put a thin layer of volcanic ash all over the northern tier of states. It would disrupt the heck out of air traffic. But it would not kill us all, or wipe out our civilization. (too bad, some might say!)

A supervolcano could be bad news, could disrupt agriculture downwind and could even affect global weather patterns (which actually could help forestall global warming?!). I still don't think it would come close to killing us all or wiping out the country.