Surface Smoke Returns, Dry Conditions Through Sunday

FastCast—Thursday, Oct. 5 to Sunday, Oct. 9:

After 2 mostly cloudy days due to an inversion layer, a change is expected by Friday. Thursday’s forecast is questionable, with the potential for another inversion layer. However, this may not be the case due to light easterly flow that could erode the low clouds. The easterly flow will bring surface smoke from the Bolt Creek and White River (Lake Wenatchee) Fires back into the lowlands. Air quality will likely degrade back to the “moderate” to “unhealthy for sensitive groups” categories. This will continue on Friday and likely through the weekend. Regarding temperatures, expect Thursday’s highs to be in the upper 60s to mid 70s. Then, for Friday to Sunday, highs will reach the mid to upper 70s. Lows through Sunday will be in the low 50s. A change in the weather, with cooler temperatures and a potential chance of rain, is possible on Monday, so stay tuned!

Find accurate & hyperlocal AQI readings by clicking “PurpleAir AQI Map” on the right side of the blog.

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Continue reading the full blog below!

It’s back to smoke for Western Washington. After light onshore flow brought persistent low clouds under an inversion layer, flow will turn back offshore (easterly), and that will bring back smoke. The HRRR forecast for 4 PM Thursday is below.

By Thursday afternoon, surface smoke will be present all around the lowlands. Notice the potential for a larger plume of smoke moving off the Bolt Creek and White River Fires. This may bring significant AQI degradation from Seattle/Bellevue northward, in addition to degradation to “moderate”/“unhealthy for sensitive groups” around the whole region.

The smoke continues on Friday. The HRRR forecast for 8 AM Friday is below.

Thicker concentrations of surface smoke will likely be entrenched in the lowlands on Friday morning. AQI in the “moderate,” “unhealthy for sensitive groups,” and potentially even the “unhealthy for all” categories are possible. 

The forecast doesn’t extend far past this point, but the pattern conducive to smoke in the lowlands is expected to continue into the weekend, so stay tuned.

Accurate, hyperlocal air quality measurements can be found here (PurpleAir map). Make sure that the “apply conversion” setting is set to “US EPA.”

In addition to the return of smoke, warmer temperatures are also coming back after a brief hiatus. The NAM forecast for highs on Friday is below.

Expect highs in the mid to upper 70s across Western Washington, with isolated warmer areas (near the Cascades and Whatcom County) due to offshore flow. Eastern Washington will reach the upper 70s to low 80s, and the Willamette Valley will reach the upper 70s to upper 80s.

In my last blog, the FastCast section noted that highs on Tuesday and Wednesday would reach the 70s. However, they only reached the low to mid 60s, after sun burned through thick clouds. Why is that?

The answer is an inversion. The graphic below from a NC State University study provides a great visualization of an inversion.

Notice that under a temperature inversion, cooler air is trapped near the surface by a band of warmer air, known as the inversion layer. In most situations, clouds are present to separate the cooler air and the inversion layer. 

This is precisely what happened on Tuesday and Wednesday, contributing to lower temperatures and persistent clouds and fog. This pattern is most common under high pressure ridges. If we take a look at Wednesday’s UW model forecast, there is a big high pressure ridge in place over the Pacific Northwest.

Inversions are most common under high pressure ridges in fall or winter, both conditions that were present. Also, forecasts have a hard time predicting inversions multiple days out, hence the lack of accuracy in the last blog.

Stay tuned for information on the upcoming warm (and possibly smoky) weekend and the chance of rain/pattern change on Monday!