Directly from National Weather Service:
... Low snow levels very late Monday night and Tuesday morning in the lower elevations of southwest Washington and northwest Oregon...
In the wake of the very strong Pacific frontal system that was moving through on Monday... colder air will spread in and will bring another round of very low snow levels to the lower elevations of southwest Washington and northwest Oregon very late Monday night and on Tuesday morning.
Snow levels could get close to 500 feet or lower Tuesday morning... and possibly close to the valley floor in brief heavier showers. Amounts at higher elevations around the valley such as The West Hills of Portland could see one or two inch accumulations. Amounts at the valley floor should be spotty and lighter.