If this week’s smoky skies seem unprecedented, that’s not far wrong: Thanks to an unusually unfortunate weather pattern, Seattle just recorded its worst 24-hour air quality in almost two decades of recordkeeping.

University of Washington atmospheric scientist Cliff Mass called attention to the high levels in a blog post today. “I have been here a long time, and I have never seen anything this bad,” Mass wrote.

How bad is it? Mass said particulate readings from the Puget Sound Clean Air Agency’s Duwamish monitoring site have registered the worst 24-hour average air quality since the agency began keeping records in 2000.

Meanwhile, the National Weather Service noted that Washington state has the worst air quality in the nation today:

This probably comes as no surprise, but Washington has the worst air quality in the country today thanks to wildfire #smoke. Conditions improve slightly tomorrow #wawx pic.twitter.com/8jOiUesIaC — NWS Seattle (@NWSSeattle) August 15, 2018

Historically, the Seattle area’s air quality is worse in the winter than in the summer, due to smoke from firewood and other combustion products. But that pattern didn’t hold for last summer and this summer, due to the spread of Western wildfires.

This week’s weather has created a “perfect storm” of smoke for the Seattle area’s skies, Mass said. Air circulation patterns are bringing in smoke from wildfires in California and Oregon to the south, Canadian wildfires to the north, and the Maple Fire on the Olympic Peninsula to the west. What’s more, there are no strong winds to push the smoke eastward.

Mass explains the details, including a nasty subsidence inversion on the Washington coast, in his blog post.

The Puget Sound Clean Air Agency advises residents throughout the Puget Sound region to stay indoors today. “It may well be into Thursday before it is noticeably clearer,” the agency advises.

As bad as it is in Seattle, it’s worse in Central Washington, where wildfires are raging. The Environmental Protection Agency’s AirNow monitoring program currently rates Wenatchee and Chelan as the worst sites in the country for air quality, with sites in Oregon and California filling out the top five.