Will Puget Sound country have another major smoke episode, as we had during August of 2017 and 2018?
Several media outlets are already talking about an above-normal wildfire summer for our region (see below), based in part on the below-normal snowpack in the north Cascades, the warm/dry conditions of the past few weeks, and heat wave in late March that resulted in dozens of small fires in western Washington. Some are warning about more dense smoke.
My take? I believe that although it could happen, it is most probable that will escape the dense low-level smoke of last summer.
To put things into perspective, here is the daily maximum small particle (PM2.5) concentrations during summer since 1990, produced by Erik Saganic of the Puget Sound Clean Air Agency. An IMPROVING trend from 1990 through 2016, which might expected from cleaner cars and trucks, as well as other technological improvements. But then we have two extraordinarily bad summers in a row--I mean in a different world from the previous decades.
Is this the new normal? Caused by global warming? Should we expect the trend to continue this year, resulting in the worst air quality of any major world city, as occurred last August? (see plot).
I think this is unlikely. And I believe we were very unlucky last year with fires and winds happening to align to inundate Seattle with dense smoke. Global warming is a long-term trend. Why would a downward trend be followed by a spike two years in a row, if global warming was the main driver?
Let's start with an interesting question. Do more regional wildfires mean more dense low-level smoke in Seattle?
Here are some figures from a previous blog of the area burned over Washington State from 1992 through 2017. It really correlates poorly with smoke levels over Puget Sound. There was a BIG, BIG spike of wildfire areas in 2015 (a very warm, dry spring/summer that year), but only a slight increase in the worst smoke day. Much less area burned in 2017, but much more smoke. The correlation is poor.
The truth is that most of the smoke from British Columbia, eastern Washington, and Oregon doesn't get to us at low levels for some basic meteorological reasons, including the blocking effects of the Cascades and the typical onshore flow into western Washington. And to get really smoky in Seattles, the fires have be close...either in western WA or in the nearby Cascades.
Even during the last two "smoky: summers, MOST of the time our low-level air quality was fine, with the long-distance smoke staying aloft. If you doubt this, here is a plot from the wonderful Puget Sound Clean Air Agency site showing you the maximum 24h small particle concentrations. Most of the summer had decent air quality, with the exception in mid-August. August 21st was the worst day.
What happened? A "perfect-smokestorm" and unusual set up. Fires really close to us in the north Cascades and strong easterly winds from the perfect direction to inundate Puget Sound with smoke (see image).
Very unusual. You need major fires in just the right location, plus the perfect configuration of winds, to get the smoke levels of last summer.
What about the 2017 smoke season? As shown below, the worst smoke was limited to about a week in early August, with August 3-5th being the worst.
Why so bad then? Fires to the northeast of Seattle with simultaneous strong northeasterly flow (see satellite image from August 5th).
I believe that we were unlucky two years in a row...and the unusual nature of the combination is shown by the lack of such events in the previous decades, even with similar numbers of fires and even greater fire area.
The question is not whether there will be wildfires in the region. There will be, particularly with our overgrown, poorly managed eastside forests and increasing human habitation and activity in them. The question is whether the perfect conditions will occur a third year in a row to push a dense plume over Seattle.
Yes, we might get two sixes on the local atmospheric dice for a third year...but I suspect the odds are against it.
from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/05/will-puget-sound-experience-another.html
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