Final answer:
The western region of the Midwest is arid due to the 'rain shadow' effect caused by mountain ranges that block moisture-laden winds from reaching the area.
Step-by-step explanation:
The western region of the Midwest is dry because it falls in the rain shadow of the mountains. The rain shadow effect is a phenomenon that occurs when prevailing winds carrying moisture rise up the slopes of a mountain range, cool, and drop their precipitation on the windward side. This results in the leeward side, or the side of the mountains away from the wind, receiving very little precipitation, leading to dry or arid conditions, like those seen in the Great Plains and beyond the Cascade and Sierra Nevada ranges.
As these air masses travel and rise over the mountains, they lose most of their moisture content before reaching the eastern slopes. Thus, regions like eastern Washington, eastern Oregon, and the state of Nevada experience semi-arid to arid conditions because they are placed in the rain shadow of significant mountain ranges. The rain shadow effect is also responsible for creating deserts such as Death Valley in California and affects climate patterns around the world, including in western China on the leeward side of the Himalayas and the western United States.