Eastern Oregon can get very warm in August (highs in the 90s are common), although has *very* low humidity compared to Alabama (15% or less)! Overnights can get chilly, especially the southeastern corner (high desert).
Be sure to drink a lot of water! If you're not used to high-temp, low-humidity climates, it's very easy to get dehydrated. Also, the bottom right corner is very desolate, so keep the other eye on your tow vehicle.
Here's a great website for you to find some interesting stops:
http://www.traveloregon.com/
Have a great trip!