Smoke Ready: When does smoke usually arrive in Washington?

We're in the middle of Smoke Ready week, and perhaps you might have wondered, "Why is Smoke Ready Week in mid-June?" As it turns out, there's a good reason for it. Today, we'll discuss when smoke has impacted Washington in recent years and the weather patterns that made it possible

Pictured below are tile charts from the EPA showing the maximum AQI values in two areas of the state, Seattle-Tacoma-Bellevue and Spokane-Spokane Valley. Each tile represents a day of the year with the years increasing from top to bottom. Months are labeled from left to right at the top.

During the winter months, it's not uncommon to see Moderate (yellow) air quality due to residential, industrial, and transportation pollution. You might also notice a spike from fireworks around Independence Day. But typically, air quality from April through June ranges from Good (green) to Moderate.

Since 2010, the earliest Unhealthy (red) air quality day due to wildfire smoke in either area was in 2014. Air quality around Spokane and much of northeast Washington deteriorated on July 19th during the early stages of what would become the Carlton Complex Fire near the Methow Valley. 

In 2020, air quality remained mostly Good throughout the summer on both sides of the state until September when a prolonged period of Very Unhealthy (purple) to Hazardous (maroon) conditions impacted the entire state. 

Perhaps the biggest outlier is 2022 when an exceptional smoke event impacted the state well into October. Perhaps the most surprising part of this event was the disparity between western and eastern Washington. The Seattle area saw a prolonged period of Unhealthy to, at times, Hazardous air quality. In comparison, eastern Washington saw only a few days of Unhealthy air. This was due to several large fires burning in the Cascades combined with a strong east wind event.

With these Unhealthy air quality events in mind, we can look to the past to see what weather patterns make smoky days more likely. First, let's look at Spokane's case using the dates of Unhealthy (or worse) wildfire smoke events. We'll omit the elevated AQI values around the Fourth of July for each of the following examples. Pictured below is how the upper-level weather pattern on those days compares to the 30-year average from 1991 to 2020. 

Notice the orange and red sitting right over Washington. This indicates above normal conditions, or an upper-level ridge of high pressure. Now, let's venture down to the ground look at Sea Level Pressure.

You can see orange and red in Montana and Wyoming indicating high pressure and blue offshore indicating low pressure. Since air flows from high to low, this would indicate Spokane tends to get smoky when a ridge is overhead and winds at the surface are out of the east-northeast. 

Analyzing Unhealthy air quality days in the Seattle area reveals a similar pattern, but there are some key differences.

On average, the upper-level ridge on Unhealthy air quality days for the Seattle area is centered just north of the state. Overall, not so different from Spokane.

The surface weather pattern, however, is more revealing. Notice for Seattle's case, the darker shade of blue hugs the coastline while, for Spokane, it's further offshore. As such, the change in pressure across the Cascades is greater. This suggests that, for widespread smoke to occur in Western Washington, a moderate to strong offshore wind event is almost always needed. This pattern in the summer typically results gusty east winds and lower relative humidity along the west slopes of the Cascades. You might hear your local meteorologist talking about a "thermal trough" during these events.

In summary, smoke season typically starts earlier east of the Cascades. The source of the smoke can vary, but most often impacts the surface when high pressure is overhead and surface winds are out of the east. In western Washington, smoke is more likely once the calendar flips to August and typically requires a stronger offshore wind and active wildfires in the Cascades.

This is why Smoke Ready Week is held in June! Typically, this gives us at least a month to gather supplies and make a plan. That way, when "thermal trough" and "upper-level ridge" enter the forecast and fire is on the landscape, you're already Smoke Ready!