Western Washington can thank its proximity to the Pacific Ocean as a natural air conditioning. But the Cascade Mountains act like a wall that keeps the cool air from getting over to the other side, leaving them to bake in the hot summer sun. And even on our hottest days when we get air blowing over from Eastern Washington, the water around here keeps us from getting over 100.
It can work in the opposite way in the winter. Sometimes, arctic air can filter down into Eastern Washington, but it won't be able to jump the Cascades into Western Washington, keeping us in the 30s and 40s while Eastern Washington shivers in the teens and 20s.