In the Pacific Northwest, our weather is often directed by two “forces”: our varied topography, between mountains and lowlands, and the vast, cool Pacific Ocean to our west.
The water currents of the Pacific flow in a general clockwise direction across the northern hemisphere.
That means that the waters off our coast generally have come south over time from the chilly Alaska and British Columbia coast. Air flowing over these cold waters onto land keeps us cooler in the warmer spring and summer seasons than points inland and east of the Cascades.
During the heat spike this past weekend, the wind flow from the Pacific, called “onshore flow,” is expected to shut down. Without that wind which I call “nature’s A/C”, temperatures under hot sun were able to warm into the 70s and 80s.
In fact, we had some modest “offshore flow” during these warm days, where the wind flow was generally from land out to sea. This even allowed temperatures near the coast to spike into the 80s!
The onshore flow that increased on Monday was only strong enough to cool the coast and parts of the Olympic Peninsula. The ocean breeze strengthened Monday night and Tuesday, and the cooler air made it well inland to Puget Sound.
The onshore winds also carry moisture the breezes pick up above the Pacific waters, including plenty of cloud cover. Once that cloud cover gets into the Puget Sound region, it can become trapped against the Cascade Mountains.
And if the cloud deck, or “marine layer,” is thick enough, the sunshine might not be strong enough to erode the cloud cover, while in the mountains above the clouds, there’s plenty of sun.
That’s what happened for most of Wednesday and will likely be the case Thursday and Friday.
By the time we get to the upcoming Mother’s Day weekend, the westerly onshore breeze won’t be as strong, and with fewer clouds and more sunshine, temperatures will rebound into the 70s.
However, I don’t see temperatures in the lowlands west of the Cascades getting back into the 80s anytime soon.
So, when we have big temperature swings in the spring and summer months in Western Washington, the wind direction is usually a primary factor!