In the Dec. 4 windstorm, strong high pressure in Eastern Washington pushed air though the Cascade passes toward lower pressure offshore. As the air went through the pass, it accelerated as it squeezed through.
With the Jan. 6 storm, we once again had the air rushing through the passes from strong pressure differences like last time.
Only this time, it was 15-degree air blowing from Eastern Washington. That kept the foothills entrenched in a cold wind while the rest of the area slowly warmed. It brought blowing snow at first, and then ice storms as the raindrops froze into ice as they hit the colder layer of air blowing in near the surface.