"prevailing winds blow from west to east across most of north america. winds that blow onshore from the pacific ocean are forced up the windward slopes of the cascade mountain range in the pacific northwest. hence, the heaviest precipitation falls on the slopes of the cascades."

The heaviest precipitation falls on the western slopes of the cascades.

Explanation: the wind blowing from Pacific ocean contains a lot of moisture. While it raises  it cools down and condenses forming rain. This happens on the side of cascades at which air rises. In this case it is western slope of the cascade which is closer to Pacific ocean.

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