[quote=@Lugubrious]
That'd take some serious patience.  Indigo is the most proficient magician in the guild, and she's not planning on using any other spells for the duration of the event.  Simply keeping the tornado going doesn't take as much magic as getting it started (after all, fires started by magic don't require the pyromancer to keep feeding the fires to make sure they don't go out) so she can go for hours.  Not much more than that, but still.
[/quote]

Bit of a logical fallacy there.

The fire burns continuously because it has fuel. Wind does not continuously shift, if it did, we would have tornados that never died. Obviously she could keep it going longer by conserving magic (ie energy), and I agree that it would take less magic to sustain it then start it. I just think your analogy is off. A better example would be stirring a thick shake (since that is, essentially, what is going on.)

...fuck it, Newton's first law pretty much explains it.

Edit: In retrospect, that is a pretty massive spell she has going. Believe it or not, Air is [i]heavy[/i], and moving that much at that speed...damn.

Edit 2 [url=http://www.dailymail.co.uk/news/article-2328834/Oklahoma-tornado-600-TIMES-powerful-Hiroshima-atomic-bomb.html]Here[/url] is an interesting, and relevant article.

Now, I'm not saying it's impossible, we are dealing with a fantasy world with fake physics, but by any logical means, such a spell would be very magic consuming and relatively short lived.

Although, the idea is awesome enough to be handwaved by the Rules of Cool.