We are now experiencing the most sustained extension of summer weather into autumn in many years, with one of the symptoms being the longevity of some smoldering fires over the region.
And the smoke from these fires reveals some interesting aspects of local meteorology and the effects of terrain.
Yesterday afternoon one could see smoke plumes from the Bolt Creek and Lake Wenatchee fires heading westward toward western Washington. Solar heating during the day reinvigorated the fires and helped mix the smoke vertically where it was caught by the easterly (from the east) winds of the lower atmosphere.
NASA MODIS Imagery around 1 PM, Saturday
Overnight, as the surface and lower atmosphere cooled, vertical mixing was greatly weakened and the cool air tended to move down-valley towards lower elevation...on BOTH sides of the Cascades.
To illustrate, take a look at the visible satellite image around 8 AM this (Sunday) morning.
Smoke from the Bolt Creek fire was moving westward down the Skykomish Valley into western Washington, while the Lake Wenatchee fire smoke followed the Wenatchee River Valley eastward into Leavenworth and the Wenatchee area.
Both of these "valley smoke flows" spread out as they gpt out of the mountains.
The smoke not only undermines air quality, but also affects clouds. Look carefully at the Snohomish River valley east of Seattle--can you see the fog there?
This fog is enhanced by the smoke from the Bolt Fire, whose smoke provides small particles upon which fog droplets can form.
The PurpleAir air quality observations clearly show the unfortunate implications of the smoke (see below). The Bolt Creek smoke plume is exiting around Monroe and then swept southward over the eastern suburbs of Seattle. MUCH better air quality near the Sound.
The Wenatchee River smoke plume is associated with very bad air quality from Lake Wenatchee through Leavenworth to the Columbia River.
Tomorrow (Monday) will be similar...sorry.
The Problem
The atmosphere is stuck. The atmospheric circulation has been locked into a very stable pattern with a ridge of high pressure over the West Coast and two troughs on both sides--called an Omega Pattern.
Take a look at a series of upper-level maps (500 hPa, about 18,000 ft) showing heights above sea level (you can also think of this as the pressure at 18,000 ft). Red colors indicate ridging/high pressure and blue/greens are the opposite.
This morning there is a ridge over the western US and troughs on both sides. An omega pattern).
The situation late Thursday is similar with even higher amplitude. This means dry and warm over the Northwest.
And the forecast for October 12th is simply extraordinary. An omega pattern on steroids.
We may not see a single drop of rain before mid-month. Have we ever had a completely dry first half of October?
Here is the data for Seattle....the answer is yes... several times.
Strangely enough, this hot, dry pattern will prevent new wildfires over from western Washington and Oregon, as well as California.
Why? Because it is not the pattern that will produce strong easterly winds over the Cascades--that requires a pulse of cool air and associated high pressure immediately to our east.
Atmospheric Sciences 101
Like last year, I am teaching atmospheric sciences 101: a general introduction to weather and climate, this fall. You can learn more about the class on the class website. I talk about everything from the basics of the atmosphere to weather prediction, thunderstorms, hurricanes, and local weather to global warming and climate.
I will be teaching the class in person at the UW, but will also make it available over zoom. Thus, folks can take it remotely.
If you are over 60, you can take the class through the ACCESS program for a very nominal charge (something like $15). Last year I had over 125 folks do so.
So if you are a UW student looking to learn about weather or a non-student interested in the topic, I welcome you to join me this fall. My first class is on September 29th.
