"Tomorrow is warmer than today."
The English sentence logically makes no sense as in the present tense. How about "Tomorrow will be warmer than today" or maybe "Yesterday was colder than today"
Either of the English sentences are correct for the following reasons:
- The given "Tomorrow is warmer than today" follows respectively to one's certainty that tomorrow is going to be "warmer" (but not necessarily definite).
- Your sentence best flows with the logic.