Prophecies & Edicts

'Prophecy' can refer to two different phenomenon. One is predictive- an extrapolation of known information in order to make a prediction about future events, commonly tied Storms invocation. The other is assertive: an Edict triggered under very particular circumstances which pushes events in a certain way.

Edicts (Assertive)

Fundamentally, an assertive prophecy is a bundle of Celestial Weight which can be distributed to and recalled from those that meet certain criteria, generally only affecting a certain region. Fulfilling these conditions therefore alters their Significance and subtly changes the way they interact with the world. For example, a prophecy that says two redheads will fight on a moonless night will bump up the Significance of redheads in its jurisdiction on said nights, more so to those already predisposed to combat.
Practically speaking, prophecies aren't all that useful due to their inherent vagueness and inefficiency. They only help nudge events along until one such event occurs, not directly enforce an outcome on a predictable sequence of events. Almost all prophecies are holdovers from pre-Age of Myths, mostly through sheer intent (and generally, spite) from greater entities manifesting itself subconsciously more than any concerted effort. The angry curses of a bitter god are exactly the type of thing that creates a prophecy simply by being uttered, requiring little effort on the deity's part to coalesce.
Key to assertive prophecies is that they, rather counter-intuitively, grow over time. This is what differentiates them from the typical mortal weavings of Celestial energy, formed from the perspective of the material world and its laws. Instead of slowly dissipating, they instead accumulate more and more weight until eventually fulfilled.
It's unheard of to deliberately create prophecies with the intention of creating an exact situation; they're far too inconsistent for that. In fact, many such edicts are deliberately designed to be (near) impossible to fulfill, instead acting as a constant nudge towards general patterns. Utilizing the general weight and probability manipulation mechanism of an edict without actually expecting it to truly complete is a common way for greater entities to impose their will on the world.

Storms (Predictive)

Future sight. The Plane of Storms is intrinsically tied to knowledge, and consequently has an association with prophecy. In theory, it does actually grant the requisite components for the ability: however, it only works if one has a total monopoly over Storms (impossible in the modern eras, achieved at most for a fraction of a moment in past ages). Otherwise, every instance of prophecy disrupts others and none of it works, making the concept largely useless.
That being said, prophecy still exists for things which can be cleanly extrapolated with no hope of human intervention. For example, the passing of a comet, or movement of other celestial bodies.