A dense smoke plume from large wildfires in Alberta has reached the Pacific Northwest.
The view from the Seattle PanoCam, looking northwest around 7 AM this morning, clearly shows the southern edge of the smoke cloud.
And the visible satellite imagery around the same time displays the massive smoke cloud extending southward over Washington State. You can also see low clouds over the Pacific, which extended inland into southwest Washington.
As the smoke cloud moved in, visibility around Seattle has declined and the Cascades are now in the haze.
Why is our air quality decent, with lots of smoke in the vicinity?
Because the smoke is overhead and not mixing down to the surface!
To demonstrate this, below are plots of ceilometer data from a unit in Marysville, WA deployed by Puget Sound Clean Air Agency. Ceilometers shoot lasers vertically and can measure the amount of smoke overhead.
Time is on the x-axis and increases to the right. You can see the dark area between 2500 and 4500 meters above the surface. That is the smoke. This is high enough that there is a good chance it won't mix down to near sea level.
A classic situation. Distant wildfires produce smoke that can move in overhead but are too high to be mixed to the surface as the ground warms and the lower layers of the atmosphere convect (moving up and down from surface heating).
So why did we get a shot of the Alberta smoke? Because the lower atmosphere winds were just right!
Below are the winds at around 5000 ft (850 hPa pressure). Winds from the north over Alberta and eastern BC, become northeasterly (from the NE) over Washington State. Perfect for moving Alberta smoke into the Northwest.
Why the smoke now?
There is a lot of talk about the smoke situation, with some suggesting that global warming is the cause.
To understand the situation, consider that late spring is NOT an unusual time for large wildfires in northern Alberta. For example, there was the May 1, 2016, Fort McMurray wildfire and the Flat Top Fire during May 11-15, 2011. And there are many other examples I could cite of major Alberta fires in spring.
Importantly, the fires were set up by REGIONAL warm/dry conditions associated with anomalous high pressure over Canada this spring.
Let me demonstrate this to you. Below is the difference from normal of heights (like pressure) at 500 hPa pressure (about 5500 meters above sea level for March 15 to May 15. Do you see the yellow to reds over southern Canada? That is higher than normal heights and pressure, which results in warm/dry conditions.
The ridging/high pressure set up the fire. Also note that the anomalous upper-level pattern was associated with troughing (lower than normal pressure) over the West Coast, giving us a wet/cool pattern--particularly in Californian).
So the wet conditions along the West Coast and dry/warm conditions in Canada can be traced to the SAME anomalous upper-level pattern.
