Some ash from Mt St Helens went west, most went east.
A bigger explosion may result in more stuff being deposited west, but it's likely that most will go east. Not all, but most.
The point is, no scientist has ever observed a supervolcano eruption. We can only surmise just what goes down during one in regards to how the ash is propagated. But the geological record is clear that that ash WAS propogated great distances at right angles to the prevailing wind. One can look at the map and see that the wind does create a kind of bow shock, as the ash did not progress far into the wind direction. But the ash did go nearly a thousand miles at right angles to the wind direction, and that isn’t speculation, it is measured fact.
Some ash from Mt St Helens went west, most went east.
A bigger explosion may result in more stuff being deposited west, but it's likely that most will go east. Not all, but most.