Why the morning fog won’t quit across Western Washington

The start of October has been gray and gloomy in the lowlands of Western Washington, with morning fog and low clouds only slowly breaking in some spots through midday.

With no significant storm systems nearby and high pressure in place, we’re in a stagnant weather pattern with a pronounced temperature inversion.

Normally, temperatures cool as you move higher in the atmosphere. But when a dome of high pressure settles over the region in the fall and winter, that usual temperature profile can flip.

Even if temperatures just a few thousand feet above the ground are only a few degrees warmer than surface temperatures, the colder air near the ground becomes trapped.

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Because cold air is denser than warm air, it tends to settle into the lower elevations. In the Puget Sound region, where the lowlands are surrounded by mountains, that colder air can remain locked in place for days.

Fog and low clouds can be stubborn to clear, often requiring daytime heating from the sun to mix the air, and pollutants can become trapped beneath the inversion as well.

We’re already seeing some reduced air quality in parts of the South Sound on Monday evening, and I expect conditions to deteriorate somewhat over the next few days if this weather pattern persists.

To break up the inversion and end the morning fog routine, we’ll need a weather system arriving from the Pacific late Friday into Saturday.

This system will bring cooler air aloft, helping to erode the inversion, along with increasing low-level winds that should help flush out some of the trapped pollutants across the Western Washington lowlands.

The weekend weather system will also bring the first meaningful chance of rain for many locations so far this month.