late 14c., "to scheme," from fore- "before" + casten in the sense of "contrive, plan, prepare" (late 14c.; see cast (v.)). Meaning "predict events" first attested late 15c. (cast (v.) "to perceive, notice" is from late 14c.). Related: Forecasting.
Whether we are to say forecast or forecasted in the past tense & participle depends on whether we regard the verb or the noun as the original from which the other is formed; ... The verb is in fact recorded 150 years earlier than the noun, & we may therefore thankfully rid ourselves of the ugly forecasted; it may be hoped that we should do so even if history were against us, but this time it is kind. [Fowler, 1926]
early 15c., "forethought, prudence," probably from forecast (v.). Meaning "conjectured estimate of a future course" is from 1670s. A Middle English word for weather forecasting (also divination by reading signs in the clouds or weather) was aeromancy (late 14c.).