My uneducated guess is that the vast majority of the time when there was snow in the Western WA/OR lowlands it was NOT because of a SSW. This board seems obsessed with having a SSW event in order to get cold and snow.
So how often does a SSW event occur? Once a year? No doubt sometime when it occurs it sends the cold air to the East. Seems to me that if the PNW Western lowlands needed a SSW event to happen at just the right place to occur (like February 2019), then it would very rarely snow.