How does the Pacific mountain system affect rainfall in western North America?

The Pacific mountain system significantly influences the climate of western North America by acting as an orographic barrier to moisture-laden storms arriving from the Pacific Ocean.1

As these air masses encounter the mountains, they are forced upward, cooling and releasing large quantities of precipitation on the windward (western) slopes. This process results in some of the highest annual precipitation levels in North America, particularly along the coasts of British Columbia and the Olympic Mountains.12

Conversely, the regions located on the eastern, or leeward, side of these ranges experience a rain shadow effect. In these areas, the air has already lost much of its moisture, leading to significantly reduced rainfall. For example, annual precipitation drops drastically immediately east of the Cascade Range, illustrating the stark contrast between the wet windward slopes and the arid conditions found in the rain shadow.1342