。
It's not the case that Hokkaido is always cold.
Literal
Hokkaido [subject-が] always cold [reason-わけ] [topic-では] not.
The pattern [clause]+わけではない ('it's not the case that, doesn't mean that') is a key contradictory hedge — denies a generalization without denying its elements. Used to correct overgeneralizations. わけ ('reason, case') functions as an abstract 'thing [clause] means.' A common correction of a stereotype.