Colorado gets ice storms on a fairly regular basis. We had one two years ago. It had been snowing lightly most of the afternoon, but the streets were warm enough that they didn't freeze. Well, right about the time that evening rush hour started, there was a sudden temperature drop of about 10 degrees. Presto--instant city-wide ice rink. And of course the plows were hampered by the rush hour traffic, so things were a major mess for a long time.