I would think if hitting the northwest, Rockies, high elevation crossings are on the trip iteniary, that August would be the month. No experience with doing this, but believe I remember seeing snow in the Rockies in September as being a common occurance.
Last year we did a 2000 mile run up the east coast to NH, VT, NY, PN, MD and VA. It was 9 days and didn't rain one day on us. We did have to out run it in upstate NY but we stayed dry.