It’s almost too late in the year for this to apply to most of the Southeast, but I thought this blog post by Cliff Mass of Washington State was a good discussion of why cars and other surfaces can accumulate frost even if the air temperature is above freezing. The same physical principles apply in our region.  You can read it at https://cliffmass.blogspot.com/2017/04/frost-on-your-car-but-temperature-is.html.

Source: Commons Wikimedia