I'm thinking of doing a story oriented game, and was considering getting a start on the writing and maybe even concept art beforehand, so the Jam can be taken up by making the actual game. What do y'all think? Is it in the spirit of the Jam?

Totally, especially for one like this.  Game jams are a great place to practice tweaking your workflow to see what works best for you.  If you're excited to get started on writing and creating concept art you totally should.  I may start doing to same this week :)

Yep! Go for it! This challenge is not about finding an idea or proving that you made it all by yourself during the duration of it, it's to refine your scoping, planning and execution skills while preventing the crunch that often happen if those things are badly managed. So yeah prepare yourself, think about what you'll work on, when and how, it's totally okay. I even think that for this one it's better to start with a clear idea/concept of what you'll work on or even a started project than to start with nothing.

It's all about no crunch, so I think preparing for the Jam is a good way to prevent crunch from building up, as crunch times definitely hit hard, even if you don't notice it at first.