Why?
Aren't both the jetstream and the prevailing "weather" from the west (flowing easterly?)
Exactly.
Everything East of Yellowstone will be buried under feet of ash as far West as Chicago and it will taper off going further East.
And of course, we can expect the Sulfuric Acid in the upper atmosphere to trigger a new ice age or at least very extended winters for several years causing repeated crop failures. Millions will die of starvation.
Everything west of the Mississippi up to Yellowstone, Wyoming and somewhat beyond. Below is link to images of YS ashfalls from the 3 major eruptions starting around 2 million years ago. My father wrote an unpublished novel about a major YS eruption in current times. In it he had over 6 ft. falling in Denver. Probably 3 ft. at MS.