Fire and Wind

If you told me that a Labor Day windstorm brought 12 consecutive hours of sustained hurricane-force winds to Timberline ski resort on Mt. Hood, I would have burst out laughing. “No way!” I would have said. “There’s no way that could ever happen in early September.” At least, that is what I would have thought before the Labor Day wind/firestorm of 2020.

On Labor Day afternoon (9/7), an intense, extremely dry cold front raced across the Columbia Basin, creating dust storms so intense that they could clearly be seen from satellite. Winds picked up over Western Washington throughout the day, and this cold front slammed into the Willamette Valley in the afternoon, bringing sudden, intense winds accompanied by dust, cooler temperatures, and much, much lower dewpoints. These extremely strong, dry winds started new forest fires by downing energized power lines and growing abandoned, smoldering campfires into raging tempests, and they turned existing wildfires into hellish beasts that devoured everything in their path.

When the dry, easterly winds finally shut off on Thursday 9/10, Oregon wildfires had scorched over 400,000 acres, and several towns in the Cascade foothills had been burnt off the face of the Earth. Thankfully, the fires over Western Washington were far smaller and less damaging than those in Western Oregon, though the Big Hollow fire south of Mt. St. Helens burnt nearly 25,000 acres during the event.

And then there was the smoke. The 2010s have seen plenty of smoky summers, but I’d never tasted air as noxious as it was this September. The smoke cooled temperatures so drastically that we became enshrouded in a thick, persistent layer of fog. But as bad as our air quality was, it was pristine compared to that over the Willamette Valley, which remained in “hazardous” conditions for nearly a week following the firestorm.

The 2020 Labor Day fires were a textbook “worst-case-scenario” fire pattern for the Pacific NW. In this pattern, a deep upper-level trough plunges south from Canada and spills cool, extremely dry air into the Columbia Basin. This upper-level trough also helps push a tongue of warm air offshore, creating a very strong thermal gradient. This thermal gradient induces a strong pressure gradient - and very strong, extremely dry, easterly winds.

This pattern occurs occasionally in October and frequently occurs from November through February. However, it isn’t associated with high fire danger for the Pacific Northwest then because our fuels are very wet, and the air is frigid. Our fuels are plenty dry during July and August, but this pattern doesn’t occur because the Canadian airmass is not cool enough to create the intense offshore gradients needed for truly extreme fire danger.

But in September, our fuels are still plenty dry (in fact, they are often the driest of the entire year) and the arctic has cooled enough for strong cold fronts to plunge southward into the states. And on rare occasions, these cold fronts can bring California-style fire danger to the Pacific Northwest.

Despite the intensity of this event, it was not unprecedented. The Yacolt Fire of September 11, 1902 traveled 30 miles in 36 hours from Carson, WA along the Columbia River Gorge all the way northwest to Yacolt, WA. Seattle’s streetlights glowed at noon, and the blaze dropped a half inch of ash in Portland. Another devastating Columbia River Gorge fire occurred on September 15, 1929, and on September 20, 1951, a fire broke out in Forks, WA on the Olympic Peninsula and traveled an astonishing 18 miles in only 6 hours. The 2020 Labor Day Fires are just the most recent occurrence of this devastating pattern.

Even so, the 2020 Labor Day Storm was likely particularly intense, even for a “worst-case” pattern. Unfortunately, wind speed and humidity records from those prior events are scant and difficult to find, but there is a robust record of daily high/low temperatures. The 2020 storm had the strongest cold front of all the storms - consider that Denver had a high of 93 and sunshine on Labor Day, and a low of 31 and an inch of snow on the 8th! Stronger fronts are associated with stronger winds. And when you consider the extraordinary wind speeds measured in the 2020 storm, it’s tough to imagine any event that could be stronger.

Can we expect more forest fires in a warming climate? Unfortunately, the answer is yes. Global warming is expected to intensify droughts over the Western US and make forests more susceptible to devastating blazes. However – this easterly windstorm was still an extraordinary, 50–100-year event, and there’s no reason to expect that we’ll see more of these windstorms in a warming world. In other words, we’ll have more frequent and severe fires due to higher temperatures and lower fuels, but this was an extreme, highly anomalous event by all measures and will remain a rare occurrence even in a warming world.

~Charlie Phillips

Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist. Check out his weather website at Charlie.weathertogether.net.

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