Here are some charts with pretty colors.
Mass 5 p.m.: Snow begins to roll in from the east. Mass 7 p.m.: Snow falls over Seattle proper and the West Side--less so on the East. Midnight: That's all she wrote.
Mass' bottom line:
If you are in Seattle and you get home by 4 PM you will have no problem. By 5-6 light snow should hit. Over by midnight, probably sooner. Tomorrow's commute will be fine.