Cold air has moved in behind the low pressure center; in fact, the atmosphere is now cold enough that snow can reach the surface in heavy convective showers.
The latest visible satellite picture shows lots of convection in the cold unstable air approaching the region.
The flow is generally westerly, which means heavy snow in the Cascades. But there where will be some locations like Seattle, that will be in the rainshadow (snow shadow!) of the Olympics.
Here is the UW WRF model snow forecast for the 24 hr ending 5 AM Tuesday. Some snow reaching the surface over the lowlands, but not much. Lots in the Cascades.
The threat of lowland snow doesn't end tomorrow AM, here is the predictions for the next 24h: