The weather patterns across the state of Washington in the Pacific Northwest region are extremely diverse. The climate is moist and mild on the west side of the Cascade Mountain Range. On the eastern side, it's drier, with hot summers and cold snowy winters. The climate within each side of the Cascades also varies considerably, particularly when it comes to wind and to precipitation.
Climate Variation in Eastern Washington
Much of the land east of the Cascade Mountains is arid, either high desert or pine forest. While irrigation has allowed Eastern Washington State to become one of the most fertile growing regions in the world, the region's natural foliage includes a whole lot of sagebrush. Cities just east of the mountains benefit from the rain shadow effect, which blocks rain-producing weather systems and allows for a greater number of sunny days. As you head east the rain shadow impact diminishes - the Idaho-border city of Spokane gets twice as much annual rainfall as Ellensburg, a city that sits just east of the Cascades.
The inverse tends to be true when it comes to snowfall in Eastern Washington, where the regions closer to the mountains or at higher elevations get significantly more snow.
Climate Variation in Western Washington
The topography and the large bodies of water create extremely varied and often dynamic weather conditions in the western section of Washington State. Western Washington's topography is quite complex, with the relatively-young Olympic Mountain Range occupying the Olympic Peninsula. The sea-level cities along the east side of the Puget Sound transition quickly to the foothills of the Cascade Mountain Range, which runs the entire north-south length of the state. The Pacific Ocean, which extends into the more sheltered Puget Sound, both moderates the temperature and adds moisture to the local climate.
Rain tends to get squeezed out of the clouds on the west side of both the Olympic and the Cascade Mountains. Cities west and southwest of the Olympic Mountain Range, such as Forks and Quinault, are among the rainiest in the United States. Cities on the east and northeast side of the Olympics are in the rain shadow and are consequently among the sunnier and drier locales of Western Washington.
The most populated area, which extends from Olympia to Bellingham along the east side of Puget Sound, is also impacted by varied weather conditions. Whidbey Island and Bellingham, which face the Strait of Juan de Fuca, tend to be much windier than most of Western Washington State. The Olympic Mountain Range splits the airflow coming off the Pacific Ocean. The point where the flow converges again, typically in the North Seattle to Everett area, tends to have extremely dynamic weather that can vary significantly from that just a few miles south.
This region is called the "convergence zone," a term you'll hear often in Western Washington weather forecasts.