Foretoken, presage, prognostic, omen, augury, portent are comparable when meaning something (as an event, a phenomenon, or a condition) that serves as a sign of future happenings.
Foretoken, the general term, is applicable to anything observable which may be the basis of a prediction or forecast.
Presage is applied chiefly to indications which inspire such emotions as fear or hope, dread or longing, and confidence or despair and therefore give rise to presentiments rather than serve as a basis for prediction.
Prognostic applies to an advance indication or symptom from which a skilled person can infer what is coming; it is used in medicine of a symptom or sign useful to a physician in predicting the course or the termination of an illness.
Omen is applicable chiefly to an extraordinary event or circumstance which one feels, especially under the influence of superstition, to be a promise of something to come.
Consequently, an event of ill omen or of good omen is one that is felt to be a presage of ill or of good.
Augury and omen are often interchangeable, but augury is applicable to ordinary as well as to phenomenal circumstances, and it usually suggests discernment rather than superstition in determining whether it presages good or evil.
Portent is applicable chiefly to prodigies or marvels (as an eclipse, a comet, or an earthquake) which are interpreted as forewarnings or supernatural intimations of evil to come.