IIRC, the Yellowstone Caldera is actually over due by a couple thousand years to go off and cover the continental US with ash and plunge the world into an ice age.
Wouldn't the prevailing winds and the Rocky Mountains play a part in where the ash goes? Just like how Mt St Helens had more impact east and northerly instead of up towards Seattle?
If those are accurate, sounds like Northern Idaho, North Washington (Seattle/Spokane) are safe and Portland is fucked.