You can expect a few thundershowers every afternoon, usually accompanied by high winds, torrential rain, hail, even snow. They don't last long but it rains hard while it's happening! It's a good idea to get an early morning start so you're off high, exposed places by 1 pm. Definitely you need that rain jacket. You may also wish for rain pants if it snows, which can happen any time. In August 2010, Titcomb Basin got a foot of snow, two weekends in a row!
I take both wind shirt and rain jacket. The wind shirt is not only great when it's windy (obviously) but not cold, but is also a great light wrap in mild weather at rest stops or in camp in the early evening. It also keeps the mosquitoes and biting flies off. If you try to wear your rain jacket for these purposes, you'll roast!
I still hope that I can get back there in September, if this !@#$%^&*! plantar fasciitis ever goes away! Getting better, but definitely not gone yet.