Western Washington Summer Smoke: Was 2017 the Smokiest Summer Ever? If so, why?

 This was a smoky summer in western Washington and many people feel it was the worst they ever experienced here in terms of the number of days with serious smoke.

So how unusual was it?  And if it was unusual, then why?  Global warming?  Mismanaged forests?  Unusual weather?  Let's examine this issue in some detail.

My own subjective experience was that this was the smokiest summers I have experienced going back four decades.  A number of folks who have lived here for many years suggested the same thing.   What about objective information?

The Puget Sound Clean Air Agency has measurements of atmospheric particulates (PM 2.5) that go back to 2000. Looking at daily averages at Seattle for that period, one finds that early August 2017 had the highest values for the entire record, and looking closely (click on image to enlarge), you will find that this summer was the smokiest as well.

Or take a look at the monthly averages.  August 2017 was the smokiest of ANY month...even the normally more polluted winter months.  Very impressive.

So for the younger folks around Puget Sound reading this blog, summer 2017 was the smokiest you have experienced.  This is probably true for those of you who have gray hairs.

If ones goes back 100-120 years, there were some smokier summers or at least smokier events.  For example, there was the famous Yacolt Burn, north of the Columbia Gorge, in 1902.  According to historical accounts, the smoke was so thick that street lights glowed at noon in Seattle 160 miles (258 km) away and ships on the Columbia River were forced to navigate by compass.  Much worse than this summer.

But what was it about this summer that made it so smoky? 

From I can discern from the wildfire statistics, we had a big year, but not a record-breaking one (for the past few decades) in terms of acreage of burned area over our region. 2015, for example, had more area burned.

We started the summer quite moist, with the wettest winter/early spring in regional history.   Snowpack was good.   So we began the season in good shape.

But what about the July-August weather and soil moisture conditions?  July-August temperature over WA State was above normal, but no record.

 July-August precipitation was low, but several years were in the same neighborhood.   Besides we typically get little rain anyway.

 And the Palmer Drought Severity Index (negative is bad) showed conditions ere not unusual at all.

So we were warm and dry, but why so much smoke west of the Cascade crest?

We need to dig deeper to explain our smoky summer.  One thing we did have was varied sources of fire.  First, we got smoke from BC, then southern Oregon, and then our homegrown fires on the eastern sides of the Cascades.   I can't remember a year with so many fires, so geographically dispersed, coupled with an early start in BC.

And when the smoke got it in, it didn't leave due to unusually stagnant conditions.

And in fact, the atmospheric circulation over our region WAS anomalous.  As shown by the upper level chart below (which shows the anomalies from normal of 500 hPa heights), we had unusually persistent high pressure over the western U.S. for June through September (red colors).

Such high pressure produces warmer temperatures, less rainfall, and lighter winds.... everything we observed this summer.  And a very strong LOW PRESSURE  area (trough or low) was observed offshore (the purple colors), which weakened he normal eastern Pacific High pressure, that in turn contributes to the onshore flow of clean marine air.

So my take is that the key player in our smoky summer, the progenitor of most of our problems, was the high pressure area (or ridge) that parked itself over the region, coupled with the strong offshore trough.  This combination greatly weakened the onshore flow of clean air that western WA generally enjoys.  It produced stagnation, sinking warm/dry air, and frequent bouts of easterly flow that brought in the smoke.

There is no reason to expect that such high pressure areas have anything to do so human-caused global warming, and climate models actually predict less of them as the earth warms.  But humans have contributed to the wildfire issue, with degraded, highly flammable forests east of the Cascade crest, the legacy of years of mismanagement and fire suppression. Our political leadership have been too slow to deal with our problematic forests.