Up to an inch of rain Tuesday before fall sunshine returns to western Washington

The fall season’s first rainfall occurred last week. After a weekend of sunshine, another Pacific weather system is forecast to swing onshore Tuesday for more rain, as much as an inch along the coast and in the mountains, and up to a half inch for the rest of western Washington.

High temperatures Tuesday are expected to be about five to 10 degrees below late September averages, only climbing to the mid-50s to lower 60s range.

Later in the week, higher pressure aloft is forecast to build over the Pacific Northwest for a return to dry conditions and early fall sunshine. High temperatures are anticipated to rise into the mid-60s to mid-70s.

Clear skies, along with longer nights, mean cooler nights and mornings. Lows are expected to dip into the 40s, with some of the usually cooler places like the South Sound and outlying areas dropping into the upper 30s.

During the fall season, when night skies are clear, temperatures often keep cooling even after sunrise.

Radiational cooling

As the sun rises above the Cascade crest, the air near the surface continues to cool as heat escapes — this is called radiational cooling. Early in the day, the sun angle is still too low to terminate the radiational cooling. Until the sun rises higher and hits the surface at a steeper angle, temperatures may cool a few more degrees before they finally begin to warm.

After recent rains, when fog forms, the sun has to rise higher and at a sharper angle to help dissipate it. Sometimes, the fog is so thick in late fall and winter that the sun cannot dissipate it.

Those foggy days can be bone-chilling, with daytime temperatures struggling to reach even 40 degrees.

Until then, enjoy the early fall sunshine later this week and into this weekend, after morning fog gives way.

Ted Buehner is the KIRO Newsradio meteorologist. Follow him on X and Bluesky. Read more of his stories here.