Since Yellowstone doesn't appear to be ready to trash us,

let's go to Iceland.  And no, I'm not going to try to pronounce the make of the volcano.
An intense earthquake swarm began Saturday deep beneath Bárðarbunga, Iceland’s largest volcano complex. (Here’s how to pronounce it.) As of Tuesday, the Icelandic Met Office cautions there’s no evidence yet of magma moving toward the surface or that an eruption is imminent. Still, Iceland is springing into action, which suggests the threat is real. These people know their volcanoes.

More here; note that the title is PROBABLE eruption at Bárðarbunga. Possibilities from least to worst:

1. A local fissure eruption. This would give limited ash and no high ash column (7km tops), unless the initial phase is explosive. In that case you might get all the way up to a 15km ash column.
2. Main eruption at Bárdarbunga central volcano. This would normally produce a VEI-2 eruption, perhaps a VEI-3. In this case I would say anything from a 5km to 15km ash column, and an eruption that lasts anything from a few hours to a couple of weeks. If the fresh magma hits an old pocket of rhyolitic mush (old stale magma) all bets are off, then expect anything from 10km to 30km ash column and widespread ash all over the area the wind blows to.
3. Kistufell central volcano. All bets are off on this one, we are talking about a volcano that has not erupted for the last 8 000 years. You would need a crystal ball to predict what could happen; I do not do crystal balls. I leave crystal balling to people that wears tinfoil hats.
4. Rifting fissure eruption. Forget flying for half a year. I am here stating that this is unlikely, but I am giving this as the improbable end parameter.

As some of Icelands volcanoes have a history of causing bad- REALLY bad- effects over a large part of the world, it's something to watch.