One who prognosticates; a foreknower or foreteller of a future course or event by present signs.

someone who makes predictions of the future (usually on the basis of special knowledge)

The calculator wheel developed and marketed by the Purple Martin Conservation Association used to predict hatching and fledging dates, and to determine nestling ages and breeding success. As the name implies, it is a device for telling the future (i.e., prognosticate). It also has the root name Progne in it, as in Progne subis, the Latin name of the Purple Martin.