I believe Harry said, not long before St. Helens blew him to kingdom come, words to the effect that he'd lived there such a long time and buried his wife there, that if the mountain blew he was gonna go with 'er.
I don't think that, at least in the last few days, he really expected "his" mountain not to erupt or that he would survive it.
He was kind of like a captain going down with his ship.