August is often thought of as the hottest time of the summer in the Pacific Northwest, with searing blue skies and sunshine – ideal for boating or heading to a shady park with a wading pool.
That's especially true of the past two summers, when hotter-than-average temperatures dried out everything and boosted wildfires that spewed smoke into the region.
So, the rapid cool down that started Thursday comes as a bit of a shock.
Showers and cloudy mornings dominate the forecast this Labor Day weekend. Just in time for the start of school, it feels like summer has come to a sudden end, following a season that's been much cooler and wetter.
THIS IS NORMAL
It turns out, this is more normal than the heat we experienced in 2017 and 2018.
“Quite frankly, this is close to normal,” says KNKX weather expert Cliff Mass, a professor of atmospheric sciences at the University of Washington. He says what most people think of as summer conditions typically make a pretty swift exit here around the end of August.
“If you plot the probability of precipitation at Sea-Tac Airport, for instance, during the early part of August, it’s only about 10 percent. By the time you get to the end of the month, it’s up to like 25 percent," Mass said. "So it is extremely normal for showers — generally light showers — to start moving into the region late August into early September.”
This year, Mass says, it looks like Sea-Tac is registering a little bit above normal precipitation levels for the summer. Temperatures have been a little bit warmer than normal, “but not the crazy warm temperatures we saw the previous two summers.”
ALSO NORMAL: PORTLAND'S HOTTER
Another aspect of normal in the summer season, Mass says, is the consistent difference in temperatures between greater Puget Sound and Portland, where weather dynamics and the geography of the Willamette River Valley routinely push temperatures 15-20 degrees higher than here.
The middle of this past week was a classic example, with Wednesday's highs in greater Seattle in the upper 80s while they soared to about 97 in the Portland area. Mass says this is not just because Portland is south of Seattle — the distance is actually less than 150 miles.
“It’s not really the latitude that counts here," Mass said. "It’s the terrain and the water."
Seattle’s location at sea level on Puget Sound gives the region a “clear shot” for the “natural air conditioning” provided by the cold Pacific Ocean.
Portland is in the northern Willamette River Valley with the Cascades to one side, the Siskiyou Mountains to the south, the Coastal Mountains to the west.
“So they are kind of landlocked,” Mass said. “Isolated from the cool waters of the Pacific.”
Mass says there’s another, more complicated reason why we stay cooler. It has to do with a meteorological feature called the "thermal trough" that comes up from California.
The area around the Willamette River Valley stays warm with this feature, but the low pressure draws cooler air from off the ocean and from British Columbia into Puget Sound.
“So the thermal trough, which comes in from the south, pulls cool air in from the north (too),” Mass said. And that’s a source for us of “natural air conditioning during these warm periods.”
We won’t be needing much of that for the next several days.
COOL LABOR DAY WEEKEND AHEAD
Here’s a summary of the five-day forecast, Mass says:
Friday: Cool with morning clouds and sprinkles, temperatures in the low to mid-70s.
Saturday: Cloudy with a band of showers moving through midday.
Sunday: Improving. Morning clouds with temperatures reaching the mid- to upper 70s. Partly cloudy skies in the afternoon.
Monday-Tuesday: “More normal,” Mass said. Upper 70s, partly cloudy, less chance of rain.
“So the worst day of the weekend is clearly going to be Saturday," he said. "Plan for that."
Editor's note: Mass will give a talk on "The Great Storms of the Pacific Coast" in Ocean Shores next Saturday evening, Sept. 7, as part of the Coastal Interpretive Center's summer lecture series. More information can be found here.
Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.