[dragnet2]

Upper atmosphere winds account for some of the spread to the west and south.

Maybe 70,000 years ago....Today, with prevailing winds, the jet stream etc, the ash would be blown mostly eastbound, before it ever reached the "upper atmosphere" as you say.

Check out weather satellite video...and tell me which direction the ash would move from that region. (Hint, with normal prevailing winds, it's not going to head west)

[dirtboy]

Today, with prevailing winds, the jet stream etc, the ash would be blown mostly eastbound, before it ever reached the "upper atmosphere" as you say.

Think pyroclastic flows making their own distribution. If you look at the maps, the prevailing wind elongates the deposits to the SE, so wind does play a role, but the deposits can travel under their own power.

At Mount St. Helens, ten miles from the eruption, trees on the lee side of hills were snapped off and knocked over so that you could actually see the swirl patterns in the flows - and St. Helens was nothing compared to a supervolcano eruption.

[Godzilla]

Maybe 70,000 years ago....Today, with prevailing winds, the jet stream etc, the ash would be blown mostly eastbound, before it ever reached the "upper atmosphere" as you say.

Please note, the top of the eruption column has reached the upper atmosphere in this picture.

I know the following may be a little complicated for you, but here is a example showing the upper air flow paths.