Wednesday, September 5, 2018

Statewide forecast — Sept. 5: Smoke in eastern WA until Friday, west side starts to clear Thursday.

Some minor tweaks to the forecast issued yesterday:
  1. A new-ish fire near Mt. Rainier (Wrong Creek) is pumping smoke into the south sound
  2. Strong-ish winds on Friday will increase fire growth in eastern WA
  3. Seems like the clearing is being slowed down slightly.
Here's the satellite picture from this morning (click for 1-2PM animation).

Notice how smoke has filled many of the Cascade valleys. The animation shows some smoke plumes have started blowing eastward this afternoon. Problem is, surface level winds are still easterly and will likely remain so until early Thursday. It will then take several hours to clean out western WA, so expect air to remain Moderate to Unhealthy for Sensitive Groups.

It will be Friday AM before eastern WA starts to scrub out, leaving many areas with Unhealthy or worse air. As the front drags through eastern WA:
  1. Strong-ish winds will fan the flames and likely increase fire growth
  2. Some Oregon smoke could clip southern and southeastern WA, perpetuating Moderate/ USG conditions
A return to Good air is likely over the weekend, save for areas very close to large fires.


  1. Thank you for this important information. Please keep up the good work but also do not "burn out" or get overloaded. This work has refined over the years and I thank you and you agencies for collaborative efforts to keep the public informed. Thank again and again

  2. Thank you for your better coverage of eastern Washington in recent weeks. I mentioned that we needed better coverage of the eastern part of the state during a recent request for comments that you solicited recently. You must have taken that to heart!

  3. I'm sure this has been answered before (if so, feel free to just direct me to direct link...), but why the disparity of PM 2.5 between the site and Airnow currently(12 noon) has north Seattle at 89, but Purpleair shows Seattle at 150. On this same thread, there seems to sometimes be HUGE differences between the reported PM 2.5 at different parts of the city(Madrona area is currently at 66, north Seattle 150) via Purpleair. Any reason for this? Thank you!

    1. There are a couple of reasons for the discrepancy. 1) Airnow reflects an averaging of the last couple of hours, while purple air is reporting air quality at a given moment. If air quality changes drastically over a short period of time, the airnow data will be slower to catch up. 2)Several studies comparing purple air monitors to federal reference methods or federal equivalent methods show the purple air monitors tend to overpredict PM2.5 in the atmosphere. Purple air monitors base their measurement on the amount of light scattered by particles in the atmosphere and the previously mentioned studies have raised some concerns as to the accuracy of the calibration.

    2. Please scroll down to the comments following this post and read about the known over-estimation of PA samplers:

      AirNow reports a single value for "Seattle", based on the worst monitor in the metropolitan area.

  4. PurpleAir is aware of elevated readings and offers an AQandU correction factor that lowers their numbers on their monitor map site. AQandU = Courtesy of the University of Utah, correction factors from their study of the PA sensors. The AirNow sensor near me always reads (in my opinion) lower than actual visually speaking and lower than PurpleAir w/correction factor applied. When I can't see further than 5 miles for the whole day, that can't be in the 'good' (green) 0-50 AQI scale that AirNow monitor indicates for that same time duration period using the 5-3-1 method and/or what purpleair had indicated as being in the 'orange' 51-100 scale for comparison.


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