Final answer:
The statement is false because it's actually the eastern side of Washington that is generally warmer due to the rain shadow effect of the Cascade Range, causing a semi-arid climate.
Step-by-step explanation:
The assertion that it is generally warmer on the western side of Washington is false. In fact, due to the rain shadow effect caused by the Cascade Range, the eastern side of Washington State experiences a semi-arid climate. This effect occurs when moisture-laden air masses move in from the Pacific Ocean and are forced upward by the mountain ranges. As they rise, the air masses cool and release most of their moisture on the western, or windward, side of the mountains. This leaves the eastern, or leeward, side much drier and often warmer, especially during the summer months.
In contrast, the coastal areas and the western side of the mountains in Washington tend to have a more temperate climate with cooler summers and milder winters compared to the eastern side.