The single most important and unique climatic phenomenon in the Pacific Northwest is the "Rainshadow Effect."
The Pacific Northwest experiences a temperate marine climate, with mild temperatures and abundant precipitation. However, the topography of the region, specifically the Cascade Range, plays a significant role in creating a rain shadow effect. The moist air from the Pacific Ocean moves towards the Cascade Range and rises, causing the air to cool and release its moisture as rain on the western side of the mountains. As the air moves over the mountains and descends on the eastern side, it becomes warmer and drier, creating a rain shadow. This results in much drier conditions on the eastern side of the mountains, particularly in areas such as eastern Washington and Oregon.
Therefore, the correct answer is C. Rainshadow Effect.