If you are in the area, often professors head into their offices during the summer. If you like, you can call her office and leave a message or email her saying that if she is in this summer, you'd like to meet with her briefly to discuss the possibility of getting a rec. If she responds that she's not in, then I'd go ahead and ask her about writing it now, and send her the appropriate info (resume, transcript, and if she requests, a writing sample).
I'd get it done sooner rather than later, so if there's any snags, you can try and fix them *before* application season really gets rolling.