If you like the sunshine and heat, get out there and enjoy it on Friday. A heat advisory is in effect through the evening, with highs expected in the mid-80s to low 90s. But KPLU weather expert Cliff Mass says a cool down is coming, likely marking the start of a big transition to cooler conditions as summer in the Northwest begins to wane.
“Today is the last day. We’ve had way-above-normal temperatures. A lot of places have gotten to the 90s, even a few mid-90s. Remember, we should be around 75 here in Seattle this time of the year,” Mass said. “But it’s all going to change this weekend.”
End Of Summer Cool Down
Mass says every year, we go through a transition from the pleasant warm and dry summer temperatures; Then, all of a sudden, something changes.
In meteorological terms, Mass says it’s the breakdown of a persistent ridge of high pressure which allows the jet stream to move through, bringing with it much cooler weather and even a few showers.
“A lot of times it happens in the end of August, occasionally the beginning of September,” Mass said. “And we’re about to do it this weekend.”
He says a massive ridge of high pressure has been lingering over the eastern Pacific and into the Northwest.
“That was giving warm air aloft, offshore flow, everything we need to get these hot temperatures. But what’s happening is that an upper-level trough [of lower pressure] will move down right over us over the weekend, especially on Sunday,” Mass said.
That will cause a surge of much cooler marine air on Friday night into Saturday. Temperatures will progressively get cooler as the week begins and the chance of showers will increase.
Cooler Saturday; Possible Showers On Sunday
Mass says Saturday will be the transition day, with temperatures dropping down into the mid-70s, partly cloudy skies in most areas and lots of clouds along the coast.
Sunday will see temperatures dropping even more, “maybe into the lower 70s.” He says the clouds will be much more persistent on Sunday.
“And maybe even a few showers; So Sunday’s definitely the worst day,” Mass said.
Much Cooler Next Week
Then as we get into next week, Mass says the upper-level troughs will be very persistent.
“We’ll have a series of these developing over the eastern Pacific, and that will bring us cooler temperatures, clouds and some showers,” he said.
Mass says the long-term forecast models indicate a continuation of that cool pattern for at least the next week — which is about as far out as they are reliable.
“It looks like a trough will be right on our coast, an area of low pressure that will give us persistently cooler temperatures,” Mass said.
He says expect temperatures to stay down in the upper 60s or maybe around 70. And there will be showers on and off.
“So, a big, big transition,” Mass said.
To hear the forecast along with a discussion of the what makes Northwest summer heat more pleasant than in other parts of the country (hint: low humidity, measured by ‘dew points,’) you can click on the “play” icon at the top of this post.
The weekly KPLU feature ‘Weather with Cliff Mass’ airs every Friday at 9 a.m. immediately following ‘BirdNote’, and twice on Friday afternoons during ‘All Things Considered’. The feature is hosted by KPLU 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 a podcast of ‘Weather with Cliff Mass’ shows.