What should I wear on an interview?

I have an interview in Tucson, Az., at the end of June. It is with Child Protective Services and I live in Mesa, 120 miles away, so I will be making a drive. I am already employed by the State of Arizona in a different department. Of course the weather will probably be warm, we are talking 107-112 degrees. My husbands suggests that I wear stockings, a jacket, the whole shebang. I know I need to look professional but likewise, according to the weather. What would the most appropriate attire be? Are nice sandals, not flip flops, ok? Slacks? Or is a suit the best way to go?