It could be dry or it could rain every day.
The most common weather pattern is clear mornings followed by a series of afternoon and early evening boomers, with breaks in between, usually starting about 2-4 pm. Each thunderstorm will give you 15-30 minutes of hard rain as well as plenty of lightning.
One year in northern Colorado it rained most of every day, with a daily cloudburst (often with an inch or two of hail) arriving almost on the dot of 4 pm each day. This went on for 2 weeks! It then cleared and was dry for most (not all) of the rest of the 6-week trip. Another year, on a 7-day trip, it alternately rained and snowed the first four days. The other three days were beautiful!
At the higher elevations, frost at night is common and it can snow at any time. By late August you'll probably have ice in your water every morning. Any snow usually melts off fast, but if you're in a ticklish place or have a whiteout it might hold you up half a day. I'd plan a couple of contingency "zero" days into your schedule just in case.
Check the DriDucks at home in the shower (you might have to seal a seam) and take plenty of duct tape! It seems that the pants (under more stress at the seat and inseam) are more apt to give way than the jacket. The emergency poncho might be an idea, at least to bounce.