This is because the pressure is lower on the West side. The snow is too light to collect on the side with the wind, the East side. On the west side there is a sort of "wind shadow" where the pressure is lower due to your car blocking the wind. Since there is less wind (lower pressure) on the lee side of your car (the west side), that is where the snow collects because it is less disturbed by wind. Here is a website that explains the physics of a snow fence: and click on the link at the bottom of that page, too.