This happens when we get a strong area of high pressure in Eastern Washington and a storm tracking up the coast. When the storm's center of low pressure is just offshore to our west, that creates a large difference in pressure between eastern and western Washington.

Air will try to move from east to west to equalize the pressure, but the Cascades act like a barrier. That leaves the mountain passes as the only option for the air to squeeze through. And as the air constricts, it speeds up, making for a strong easterly wind.