Try http://myyosemite.com to get a feel for the place from the viewpoint of someone living there year-round. You may find that 4 of 5 early Octobers have been great (or not). If you find such a pattern, you can get at least better than coin-flip chances.

BTW, October is when the night fog and moonbows seems to start up. Even without roaring waterfalls (those are really spring/early-summer things), it's pretty cool: http://www.flickr.com/photos/2716334...57627828844404