Formal:
* Presage: To indicate or foreshadow a future event.
* Augur: To predict or foreshadow, especially on the basis of signs or omens.
* Forebode: To indicate or suggest (something bad) that is going to happen.
* Prefigure: To give a prior indication of something to come.
* Bodes: (often used in the negative) to be a sign of something, especially something bad.
* Prognosticate: To predict or foretell a future event.
Informal:
* Signal: To indicate or suggest something.
* Herald: To announce or be a sign of something.
* Foretell: To predict something that will happen in the future.
* Betoken: To be a sign of something.
* Hint: To suggest something indirectly.
The best synonym for "portend" will depend on the specific context and the desired tone.