mattress of snow 

the pile of snow left on top of a car after the owner(s) have cleared the windows off. This pile of snow closely resembles a mattress on top of the car and sits in the same place as a mattress would.

This pile of snow is usually left on the car by incompetent assholes who don't understand that when the snow falls off it could hurt someone.

Did you see that mattress of snow on that mini-van?

Yeah, that shit nearly hit my windshield

mattress of snow by J-Bob 92 January 12, 2011