September in Washington

In Washington State, the weather in September marks a transitional period from the warm summer months to the cooler and wetter autumn season. Average temperatures vary significantly across the state due to its diverse geography. In the western part of the state, including cities like  Seattle and Olympia, daytime highs typically range from 65°F to 75°F (18°C to 24°C), while nighttime lows can drop to around 50°F to 55°F (10°C to 13°C). The eastern regions, such as  Spokane and the Tri-Cities, experience warmer days with temperatures often reaching 75°F to 85°F (24°C to 29°C), but cooler nights with lows in the 40s and 50s (4°C to 13°C).

Rainfall in September is generally light compared to the wetter months ahead. Western Washington, which is known for its significant rainfall, begins to see an increase in precipitation as the month progresses. Seattle, for instance, averages about 1.5 inches (38 mm) of rain in September. Eastern Washington, which lies in the rain shadow of the Cascade Range, remains much drier, with places like Spokane typically receiving only about 0.8 inches (20 mm) of rain during the month. The variability in rainfall can sometimes lead to unexpected showers, but prolonged dry spells are not uncommon, especially in the early part of the month.

Air quality in Washington State during September can be a concern due to the lingering effects of the wildfire season, which peaks in late summer. In recent years, the state has experienced varying levels of air quality due to smoke from both local and distant wildfires. For example, in 2020, large wildfires in California, Oregon, and Washington itself caused severe smoke conditions, leading to hazardous air quality levels across the state for several days. Conversely, some years see relatively clear skies and minimal air quality issues. The trend in recent years has highlighted the increasing impact of climate change on wildfire frequency and intensity, which in turn affects air quality during the late summer and early autumn months.