As you can see, the adverb, even though certain adverbs can serve as a complement in certain syntactic arrangements, is optional, thus is not required to complete the meaning of the sentence, a function that complements perform.
Think of what "quite heavily" relates to. Does it modify 'it'? Or does it modify your verbal phrase "is raining"? Of course, the latter. Besides, I don't think that adverbial phrases can be complements.