When it's very cold in Eastern Washington, a lot of times that can translate to a dual freezing level in the passes.
The freezing air will work its way east through the mountain passes, keeping the areas near the gaps at or below freezing.
Yet, if you were at 3,500 feet elsewhere, it'd be much warmer. That can lead to some slick driving conditions or freezing rain. Just one more way a mountainous terrain affects weather.