There is no accurate way to tell when the rains will start, it depends on the moisture content, and the cloud cover. Morning cloud cover means later storms. Night-time cloud cover can trap heat and mean very early storms. If there is no cloud cover then they could start anywhere from 10:00 to 1pm or so, but again, it's impossible to tell. Last weekend I was off the mountain by about 1230, and that's when it started storming. You can see the pictures from the "flood" on TV, these are NOT storms that you want to be caught in, so I'd plan to start as early as you can, and be done between 10am and 12. If you can ride longer due to the wx, great, but I wouldn't plan for it.
The trail conditions were perfect last weekend. You can't ride Little Bear, Little Elden, I'm not sure about Weatherford, but I never see many riders on that trail anyways (but it makes a nice loop to do Lookout and Supermoto).