Wind direction

Probably, since this is the direction of the jet stream.

But I would be far more concerned about ocean currents at this point, because it is unlikely that there will be releases into the air but almost a given that there will be enormous releases (way beyond the releases there have been already) into the ocean. The ocean currents AFAIK go down the coast of Japan for a few miles before they meet a larger current that brings everything straight to Alaska and from there down the west coast.