Re: If Yellowstone erupted, would people in St. Louis survive?

What would happen that far away depends on the specific dynamics of the eruption. Eruptions at Yellowstone are not all of one kind and a catastrophic "supervolcano" eruption, more properly known as a caldera eruption, would almost certainly be preceded by smaller eruptions.

The next eruption at Yellowstone is much more likely to be a small dome-forming eruption, rather than a catastrophic blast. That dome forming eruption is, in turn, likely to be preceded by phreato-magmatic explosions caused by magma flashing groundwater to steam. These dome-foming eruptions and their precursors might be dangerous to people within a few hundred meters to a couple of kilometers, but people in distant cities, even cities much closer than St Louis, would probably not notice any effects at all.

If a caldera eruption did occur, there still could be a pretty big range in the amount of magma ejected and the severity of the effects. The most recent caldera at Yellowstone erupted 1000 cubic km of magma. The ash from that one was thick enough across the Midwest US that the layer is still preserved in placesThe oldest of the three. The oldest of the three Yellowstone calderas erupted nearly two and a half times as much magma (~2400 cubic km http://www.volcano.si.edu/world/volcano.cfm?vnum=1205-01-).

In general, the bigger the eruption, the more widespread the damage, including loss of life, but the impact of the eruption depends not only on its size, but other factors including prevailing winds and topography. The most lethal impacts of caldera eruptions are massive ash flows that may reach out more than 100 km, but not as far as St Louis. The Taupo ignimbrite (ash flow) in New Zealand traveled 70-90 km ( http://rsta.royalsocietypublishing.org/content/314/1529/229.abstract) at speeds as fast as nearly 500 miles per hour. For more information on Yellowstone's ash flows, including maps showing the current distribution of the tuffs (ash flow deposits), see http://pubs.usgs.gov/pp/pp729g/ The flows undoubtedly reached farther than shown on these maps, as the ends of the flows are easily eroded, but they undoubtedly did not go twice as far as what remains, so that should give you a pretty good range. However far a Yellowstone ignimbrite reached, everything in that zone would be totally devastated and probably no one would survive.

The much larger impact would be from windblown ash. Heavy ash falls (1 foot to tens of feet of ash) would occur up to hundreds of kilometers downwind from the eruption, depending on the wind. This ash fall might directly kill some people within a few hundred kilometers of Yellowstone by burial and suffocation, but it is unlikely to immediately kill people farther away. However, this ash fall will affect people's breathing and some people who are particularly susceptible could be killed by this, even more than a thousand kilometers away. This would likely include at least a small number of people living in St Louis, but a local pulmonologist (lung doctor) might be able to give you a better number. For a very large eruption it might even be possible to see slightly increased mortality on other continents, as happens now with giant dust storms from Asia and Africa.

A potentially more significant issue is the likelihood of famine because most of the agricultural land, crops, and livestock of the Great Plains would be destroyed. This is likely to lead to some number of deaths, not just in St Louis, but across the country and perhaps beyond.

This scope of eruption is extremely unlikely, which is good, because it would be a major threat to humans, planet wide. The last eruption of this scale (Toba Caldera in Indonesia) came very near to causing the extinction of humans 75,000 years ago (perhaps only a few thousand survived worldwide, as discussed in the BBC show "Supervolcanoes" http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml)