[Strategerist]

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.

If you're talking about airborne ash, the above is absolute, complete nonsense.

The surface pyroclastic flow (mud, boulders, etc.) of St. Helens extended to the North, as the lateral blast was to the north.

But the Ash fall of St. Helens from the fine ash directed upwards was completely controlled by wind direction at the time of the eruption, as it is for every single volcano in the world.

[AntiGuv]

Well, I was actually talking about all of it, but my impression was that heavier particulates fell out of the plume swiftly more or less in the direction that they were blasted, while lighter particulates billowed into a vertical plume then subject to wind currents. Perhaps I'm wrong.

[AntiGuv]

Hey, I found a link that outlines Tephra Fallout and Ash Cloud Mechanics far more accurately than my vague recall of Geology 101.