I'd say west of the Rockies, just because there are more of us humans there than on or east of the them danged mountains if we're talking about the western US. And summer is about the only time you can pretty much count on not having to deal with snow in the Rockies if you're driving.

The most important thing for me is to know WAY ahead of time when it's going to be so I can plan my work itinerary around it.