Storm of Magic

Storms of Magic are when the Winds of Magic rise from a strong gale into a howling tempest.

The cause of such raging typhoons of magic is, like all things of Chaos, mysterious and often erratic--as likely to be explained by superstitious lore as by applied logic. Some blame the wrath of the gods, others see such violent storms as opportunistic, a time of great revelry and bloodshed. Nearly every race has some theories, legends or apocryphal tales about these turbulent and dangerous storms.

Source

 * Warhammer Fantasy: Storm of Magic pg. 5 6