Notice the snow has melted on this entire roof except the eaves. This is a great example of how ice dams start. Due to heat loss through the attic all the snow melts off the roof. The remaining snow is over the colder eaves. As winter progresses and we get more snow this will intensify. as snow melts off the roof it will re-freeze over the eaves and the ice dam increases in thickness eventually backing up into the home.

The best solution for preventing ice dams is to seal the attic floor penetration to prevent heat loss. Also, properly installed ventilation will help but thats a story for another day

This effect will intensify as winter progresses.

Ice dam forms over the colder eave.