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.