A blanket of snow will insulate the ice sheet. This keeps lake ice from forming, and results in the formation of snow ice. Snow ice doesn't have a linear crystalline structure and is very unstable. An inch or two of snow has little effect, but several inches will keep the ice sheet hovering right around the freezing point. Even in 0-10 degree air, the ice sheet will be much warmer due to the water volume in the lake holding residual heat.