If did not take much rain on Saturday evening to turn Iowa's roads to a sheet of ice. The featured time series is from the RWIS site on US Highway 20 near Waterloo. The green line (dew point) tells the story of what happened to cause an icing event. Moist air was surging into the state on Saturday with air temperatures hovering just below freezing. Since the cold air was shallow and near saturation, rain was able to fall to the ground without much evaporation nor phase change and then immediately freeze as pavement temperatures were below freezing. The timing of this event late in the afternoon followed by a cold front passage both conspired to prevent most any melting over night. Cold temperatures on Sunday kept nearly all of this ice around on untreated surfaces.

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Tags:   ice   rwis