We're already getting snow in the mountains. By December, absent a historically unprecedented lack of precipitation in the mountains, you should have a front row seat for what winter brings to the Cascades.
Quick story: once taking the Starlight in mid March, we were sitting in the lounge alongside a small group from Australia who were excited about possibly seeing snow for the first time. Sure enough, going south into the Cascades, we started seeing small, rather dirty patches of snow, and they stayed glued to the windows, taking pictures of the snow. Within thirty minutes, we were deep into the Cascades, with majestic pines crowding the rails, covered in pristine snow, and more was falling. Our Aussie friends ended up deleting their first batch of 'snow' pictures...