This is quite an interesting game, though you did not intend it that way. You should fill in the blanks first. Some possibilities: (i) troubling? vexing? worrying? (ii) a season - summer? (iii) inevitable? Regarding your question, what the sentence means is "the likelihood of storms is more worrying than the predicted in increase in rainfall." The construction "not so much A as B" means that B is the more significant or influential factor. Example: It is not so much John's philandering as his physical violence that made up Mary's mind to leave him.