The record low for Seattle / Sea-Tac is airport is only 0F as per https://www.extremeweatherwatch.com/cities/seattle/lowest-temperatures, but, there can be quite a bit of variation in the Greater Seattle area depending on location and elevation. Most of the records on that list is from 1950, suggesting that we don't usually get single digit (F) cold, but sometimes the cold air can stay for weeks.
My definition of pretty damn cold is -10C or lower (around 15F or lower) which happens maybe once every 4 - 5 years here. This year we were lucky because the cold spell wasn't accompanied by copious amount of snow, but more often than not we do get a significant amount of snow with the cold, too.
