PNW temperatures drop this week as region prepares for autumn

Does it feel like the seasons are changing? After high temperatures reached the 80s and even the lower 90s in some Western Washington locations last week, and highs in the 70s over the weekend, temperatures are going to struggle to top 70 degrees this week.

Two weak upper-level weather systems are expected to enhance low-level onshore airflow from the Pacific Ocean for cooler temperatures and greater cloud cover across Western Washington this week.

The first weak weather system aloft moving inland Monday will have an upside. The weekend-elevated wildfire smoke will be blown east out of Western Washington, offering improved overall air quality and erasing the hazy conditions. This system’s downside will be marine clouds blanketing the region with cooler high temperatures.

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On Wednesday, the second weather system aloft is expected to swing onshore. This system will not only enhance the marine cloud cover but also bring a threat of light showers. By Thursday and Friday, the low-level onshore flow from the outer coast is anticipated to ease, permitting the likelihood of some afternoon clearing.

Coastal high temperatures will remain in the 60s throughout the week. In the interior, highs will range from the mid-60s to lower 70s with the exception of Wednesday, warming only into the 60s – the coolest day of the week. The greater cloud cover this week will keep low temperatures relatively mild – only dropping into the 50s.

With daylight losing just over three minutes a day as the Sept. 22 fall equinox approaches, this week will feel like some fall weather with cooler-than-normal temperatures. Average high temperatures in the Western Washington interior are in the lower 70s.

The Seattle Mariners return from a long road trip this week with the San Diego Padres and Texas Rangers in town. Game-time temperatures will likely be in the 60s and the stadium roof may be closed on Wednesday.

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Heading into this weekend, another Pacific weather system may generate a greater chance of showers with the Apple Cup and the Sounders in action. Longer-range forecast charts also point toward cooler and wetter-than-average conditions stretching toward the end of the month.

This trend in the weather certainly gives a strong sense that autumn is just around the corner.

Ted Buehner is the KIRO Newsradio meteorologist. You can read more of Ted’s stories here and follow him on X, formerly known as Twitter.