I'm a meteorologist from Northern Arizona, working out of Flagstaff, as well as occasional photo tour guide for the Coyote Buttes. Our summer thunderstorm season usually ends by mid September, leaving the area mostly dry and warm through October. You can usually count on dry conditions at this time, which makes the Wave and Coyote Buttes much more accessable than in August, where I've experienced several impassable road crossings due to water over the road/mud/etc. In Sept/Oct, high temps will likely be in the 60s/70s F. It's one of the best times to be at the Wave/Coyote Buttes - though water may not be present unless you arrive within a few weeks of the last thunderstorm... Enjoy!!
Some of my Wave galleries can be found from here:http://www.pbase.com/brianowski