What I don't understand about that Lava Creek ash pattern is that prevailing winds blow from west to east due to coriolis forces that are unlikely to change. So, how would it be distored westward?
Volcanic eruptions frequently eject debris with such force that prevailing air currents have little initial bearing on the distribution pattern. In particular if the volcanic eruption is asymmetrical (as Mount St. Helens was, by example) then the resulting ash zone can easily end up extending completely against the wind direction.