Driving from Phoenix you can travel Interstates 17 North and 40 East, passing through Flagstaff.
The first round of the storm dumped a ton of rain on me and then the sun came out: I was pretty much sure that I would have a pretty good rainbow 180 degrees away from the setting sun. So I decided to stick some more. When the second round of the storm approached, the rainbow intensified 10 times more. Needless to say, I got soaked :>
The incredible concentration of petrified wood and the multi-colored badlands of the Painted Desert makes Petrified Forest National Park a wonderful land of amazing wonders.