I thought I saw somewhere that there was an account from some dude on the Oregon coast that said it started snowing heavily after the wind shifted and the front or occlusion passed.
Overrunning at the start of the storm before it moved much North of there? Plenty of storms start as snow there with cold air in place before the low moves North/inland.
No doubt cold backwash would have been a big deal given the low track and all of the cold air in WA. A 955 low tracking south of Seattle just seems impossible. Might be impossible now.