Everything

In all cases, the mage needs to make Willpower rolls to continue shaping the elemental magnitude of a full-fledged storm. That mage is, after all, riding herd on the most powerful energies on Earth, and such deeds require force of will.

Assume that a mage who’s calling up or otherwise trying to command a large, destructive weather front needs to make a Willpower roll as the elements converge; if she fails, then the elements slip free and do whatever the Storyteller wants them to do.

Assuming she succeeds on the initial roll, the player ought to make a new roll every hour or so in game-time; if the story goes to a new scene, then that scene requires a fresh Willpower roll. This goes on for as long as the storm lasts… and if the mage fails, then the elements escape from her command as mentioned above.

The difficulty for this roll depends upon the strength of the storm in question. A simple rain-shower should be difficulty 5; a powerful thunderstorm, difficulty 7; and a full-bore tornado difficulty 9.

What happens if the mage doesn’t make the roll? Bad things. Very bad things. The specifics should depend upon the situation, especially if the player botches that roll. But again, a mage in this position is playing with the most ruinous forces on Earth, and not all of the reasons behind the expression “Do not call up what you cannot put down” apply to Umbral entities.