The latest apocalypse: The Yellowstone Supervolcano will erupt and destroy our civilization.
The US Geological Survey got tired of the rumors, so they developed an actual computer model to try and work out what would happen (while stressing that an eruption is not remotely imminent, fleeing bison or not).
They modeled the biggest eruption likely to happen (a smaller one is much more likely). Would it cause huge problems? Yes. It would deposit ash everywhere, although not to the same degree. Cities close might be covered by over a meter of ash, but if you're in the midwest, it would be a few centimeters and the east coast would get millimeters.
How bad would it be? Small deposits might reduce road traction, causing an increase in accidents. The electrical grid might be shortened out in places. Larger deposits might collapse some buildings, block sewer lines and generally cause problems. Wind blown ash would cause respiratory problems, especially in people already susceptible. It would probably disrupt food production. Oh, and it might end our worries about global warming...at least for a while (a lesser eruption at Tamboa caused 1816's "year without a summer").
And it might happen...in a few centuries. In fact, all the evidence is that Yellowstone is slowly cooling off and activity there is declining.
Even if Yellowstone does erupt, we'll get plenty of warning and chances are it will be a small event with "minimal effect outside the park itself."
So. Stop worrying. We aren't all about to be buried in meters of ash. At least not from Yellowstone.