The snow that

The snow that melted on the upper roof area will freeze as it moves onto the lower roof area if that lower area is colder than at the top, which usually is the case. Typically, the lower area of the roof will remain colder than the upper roof area, especially in the area just above the eave, where temperatures may not be much higher than the ambient outdoor air. If the outdoor temperature is well below freezing, conditions for ice dam formation are favorable.