Fall arrived in the northwest with a September that has been wetter than normal. Now, unusually early cold is expected to hit the region as an early blast of Arctic air heads our way this weekend. It’s expected to bring record low temperatures to parts of the state and snow east of the Cascade mountains. But as it gets colder, the west side will dry out.
“People need to get their sweaters out of the drawer,” says KNKX weather expert Cliff Mass. “They’re going to need them in the morning.”
He says the cold air is expected to start hitting on Saturday, when a trough of low pressure heads southward, bringing the Arctic Express in its wake.
Bellingham could see winds gusting between 30 and 40 mph from the northeast, through the gap in the mountains in the Fraser River Valley.
Eastern Washington will get cold air for sure but also a good chance of snow in some lowland areas, such as Spokane or parts of the Okanagan region of British Columbia, stretching into north central Washington. And Mass says the snow will definitely hit on the eastern slopes of the Cascades, at levels as low as 1,500 feet.
But he says the key thing that everyone will notice, are plummeting nighttime lows and the resulting cold temperatures in the mornings.
GET READY FOR COLD MORNINGS
“We’ve been used to walking out of our apartment or our house in the morning and it’s been really mild. The low temperatures have been like in the upper 50s,” Mass says.
He says that’s because we’ve had onshore flow, coming off an ocean that’s much warmer than normal. Mass notes the 2019 marine heat wave he likes to call ‘Blob Junior’ is still there, with sea surface temperatures at least 5 degrees warmer than normal.
“And as long as we had air coming off the ocean, the minimum temperatures really haven’t been able to drop to their seasonal values,” Mass says. “But now everything changes.”
He says with the cold air coming from land to our east and north, nighttime temps will drop down into the mid- to low-40s at first, then into the 30s on Monday.
SNOW ON EAST SIDE ONLY
Mass says the set-up is perfect and he expects to shatter some records for cold temps, especially in Montana, where blizzards are predicted, with several feet of snow east of the Rockies.
“In fact, if we had this pattern in November or December, I would be talking about snow here in Seattle. But it’s early in the season,” he said.
And he says as the cold hits here, west of the Cascades, the air will dry out. Even if it were to get cold enough for snow, there wouldn't be any precipitation.
That’s because it is easterly flow – air coming from the east – and the winds it creates will rise on the eastern side of the Cascade. So that’s where that’s where the upslope flow that causes precipitation will be.
But as the air descends down to the west side, that produces drying.
“It’s all because of the easterly flow,” Mass says.
Friday: lots of clouds, rain around Puget Sound likely later in the day with a convergence zone forming north of Seattle, highs in the lower 60s.
Saturday: “the big action happens” as the key trough of lower pressure moves to the south, high winds in Bellingham area, good chance of snow east of the Cascades. Low temps dropping into the 40s.
Sunday: cold, especially in eastern Washington, where there’s a chance of snow. Dry on the west side, but cold. Highs around 57-58. Lows in the low 40s.
Monday and Tuesday: Still cold. Partly cloudy skies. Lows in the 30s.
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.