How to discern a "so...as.." in a complex long sentence?
What is currently (i) _____ civil engineers is not so much a predicted increase in annual precipitation as the likelihood that many storms will come in (ii) _____ , thereby making flooding in lower lying riparian regions (iii) _____ .

What does "not so much a predicted increase in annual precipitation as the likelihood that many storms will come" mean?

Question

How to discern a "so...as.." in a complex long sentence?
What is currently (i) _____ civil engineers is not so much a predicted increase in annual precipitation as the likelihood that many storms will come in (ii) _____ , thereby making flooding in lower lying riparian regions (iii) _____ .

What does "not so much a predicted increase in annual precipitation as the likelihood that many storms will come" mean?

Answer

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.