You know the wild magic surge? Pretty much that but as a weather effect. For every minute you're in a Mana Storm, I roll a 1d20. If it's a 1, you get hit by a random effect off the table. [i]Other[/i] things might get hit too. Fortunately it's all in the ocean, but the few times a Mana Storm goes onto land oh boy, it is a calamity.