User blog comment:Xilinoc/Ch. 542 - THE BLADE IS ME/@comment-4207271-20130620040906/@comment-5398287-20130622204922

I think the rain is more symbolic than anything. Think about it: it rained when Masaki was killed (Ichigo even noted it wasn't raining on the anniversary of her death) and it rained when Ichigo confronted Yhwach. Zangetsu even says it rains when Ichigo is sad. Hence, it's just a symbol for his emotional state, much like Holden in Catcher in the Rye. Nothing to do with his Zanpakuto.