I believe the det in this sentence represents the pronoun 'it', rather than being a definite article connected to varmest: 'Where is IT the warmest in Norway?' To clarify, we could rearrange the word order to put the subject first, something like DET er varmest i Norge hvor? I (byens navn): 'IT is (the) warmest in Norway where? In (city's name).'
One could probably also ask Hvor er varmest i Norge? 'Where is the warmest [place] in Norway?' But that would change the translation slightly - removing the overt subject 'it' from the translation, just as det is removed from the original Norwegian sentence.