Final answer:
The Cascades and Sierra Nevada mountains cause a rain shadow effect, leading to dry conditions on their leeward sides.
Step-by-step explanation:
The high mountains of the Cascades and Sierra Nevada produce a rain shadow effect that keeps the interior region dry.
The rain shadow effect occurs when moist air masses from the Pacific Ocean ascend the Cascades and Sierra Nevada, condensing and precipitating on the windward side. By the time the air descends on the leeward side, it has lost much of its moisture, resulting in arid conditions in areas like eastern Washington and Oregon. This also leads to the development of unique ecosystems and climates in the shadowed regions.