Western Washington and Oregon Wildfires are Most Frequent This Time of the Year: Should We Worry?

Climatologically, the most costly meteorological disaster west of the Cascade crest occurs at this time of the year:  massive wildfires associated with strong easterly winds.

Here is a map of the big ones through 2020

And here are their dates, areas, and direct fatalities.  Late August and early September are the favored times.

Why late summer?    

One reason is that surface fuels are relatively dry after our warm, near-rainless summer.  But even more important, early September is a period when western Washington and Oregon can experience very strong, very dry easterly winds.

It is really hard to get big fires in the west when cool, moist air floods western Oregon and Washington.  Strong easterly winds (from the east) are very dry, starting in eastern Oregon and Washington.  They dry even more as they descend the eastern slopes of the Cascades.

During the late summer and early fall, high pressure can build inland (associated with cold, dense air moving southward out of Canada).   See example below.

With high pressure inland and lower pressure along the coast, offshore-directed easterly winds develop, as illustrated by the winds at 850 hPa (about 5000 ft) at the same time.

These easterly winds, and the accompanying dry air, can start and maintain fires, such as the huge Labor Day fires of 2020.

I know your next question: will this happen this year?  

I have very good news.  It has not happened yet, and model forecasts do not suggest that this pattern will set up during the next two weeks.   After that, the threat declines rapidly.

To show this, here is the forecast for meteorological conditions at Stampede Pass in the central Cascades.  No strong easterly wind periods.  Other mountain sites are similar.

Major fires on the western side of the Northwest can easily run into the billions of dollars and result in many injuries and deaths.

What about climate change?  Will that make such fires more frequent?  

Published research (some done by my group) has found that global warming will cause a DECLINE in widespread wildfires.

That is not a typo.  A DECLINE. 

Why?  Because global warming preferentially warms the interior of the continent, with the less dense, warmer air weakening the interior high pressure that drives easterly flow

Announcement

I will be teaching Atmospheric Sciences 101 this autumn if anyone is interested.  It provides a good general intro to meteorology and weather prediction.  It will be hybrid, both in person (MUCH, MUCH BETTER) and online.  Information is found here: 

https://a.atmos.washington.edu/~cliff/ATMS101Y2026.html