This is the kind of forecast where one can make big errors because there are so many degrees of freedom and things happening at once. So lets start with what we are sure about.
We are fairly certain that:
(1) a cold front is heading for us and will cross western Washington between 4 and 7 AM.
(2) some light precipitation will accompany the front
(3) that much colder air is behind the front the freezing level will plummet. The snow level will decline to only a few hundred meters above the surface very quickly and by the end of the day will reach sea level.
(4) that the mountains will get more heavy snow..at least a foot.
(5) the the freezing level is sufficiently low that the high eastern suburbs should get more snow than the lowlands near Puget Sound
(6) that the air will get colder on Sunday and will hold in until Tuesday
(7) that outflow will develop in Fraser River Valley, push into Bellingham, and give upslope snow on the ne side of the Olympics
(8) that there will be substantial rain/snow shadowing due to the Olympics
(9) that a Puget Sound convergence zone will form sometime tomorrow and produce moderate snow in the eastern suburbs and quite possibly down to sea level.
(10) that outside the convergence zone and the upslope on the mountains, precipitation will be generally absent or light after the front goes through in the central Puget Sound region.
You see, we know quite a lot. But there is plenty we don't know...when and to what degree will the convergence zone set up? Will it be cold enough for snow near sea level if the convergence zone sets up early. Those are the the million dollar questions. So you got to be ready for the white stuff. Or not.
My own take is that the convergence zone will set up by 9-11 AM over central Puget Sound and that there will be snow above 200-300 ft, with wet snow/rain down to sea level. The convergence zone will hand around for much of the day...although it could shift in position and intensity with time. Later in the day and Sunday AM, the convergence zone will shift north over Snohomish County. Anyway, the position and strength of the convergence zone is a hard one...so keep this uncertainty in mind. Watch the weather radar in the morning to see what happens in real time.
If anyone is interested, KCTS-channel 9 in Seattle-- will replay my weather program at 1 PM. And if you want to help a good cause, you might make a pledge to keep public TV going.