Why snow in the mountains rarely reaches lowlands west of the Cascade Crest
Snow levels are dropping over the Cascade mountains. Weather officials are warning that anyone planning to cross the passes before Thanksgiving should be aware of the potential for hazardous conditions. As much as a foot is expected to pile up by Wednesday above 3,500 feet, as the temperature drops.
And it will be cold enough all around the Northwest to store your extra food outdoors if the fridge is overflowing, says KNKX weather expert Cliff Mass. But don’t expect to build a snowman. Only a few snow flurries are expected in the lowlands on the west side.
“Snow here is difficult to get west of the Cascade Crest,” said Mass, a professor of atmopsheric sciences at the University of Washington.
'JUST THE RIGHT SETUP'
Mass says the combination of cold and dry weather is pretty common – that’s what’s in the forecast for Western Washington around Thanksgiving this year. And warmer temperatures along with wet conditions are not unusual around here.
“But to be cold and wet at the same time is excruciatingly hard to do,” Mass said, which is clear from conditions this year.
The cold Arctic air that causes our snow typically comes from the north, Mass says, through the Fraser River gap near Bellingham. But it takes more than just that cold air to produce snow.
“We have to get just the right setup,” he said.
The air coming through that gap needs to combine with a low pressure system that can pull moisture in off the Pacific. That air is warmer, so it needs to set up in just the right orientation above the cold air, causing precipitation but without disturbing the cold air too much, such that the cold still reaches the surface.
“And unfortunately, if you like snow, that’s not going to happen this week,” Mass said.
He says low pressure is the missing ingredient. Instead, he expects easterly flow to push the wet weather south of us, into California.
“So I’m very pessimistic about lowland snow later this week, but it will be cold,” Mass said.
ENOUGH SNOW FOR PLAY
Mass says he expects a pretty impressive accumulation of mountain snow above 3,500 feet: “You know, 6–12 inches at least.”
That’s not enough to start the downhill ski season, but it could be good for other forms of snow play.
“If you want to go up later in the week, to play around snow shoeing or maybe even some cross country skiing, it may be possible in some places. But not downhill skiing,” Mass said.
Friday: Freezing fog in the morning (watch for back ice!), burning off later in the day. Dry with highs around 50.
Saturday: Less morning fog. Mostly dry, cloudy, highs again around 50. Rain in the evening or overnight as a weather system starts moving through.
Sunday: More precipitation as the weather system continues to push through, opening up later in the day with highs around 50.
Monday–Tuesday: A much stronger system comes in from the Northwest. “Substantial precipitation” in the mountains with snow starting at 3,500 feet and dropping steadily. Less rain around Puget Sound, with progressively colder temperatures.
Wednesday–Thursday: Cold, dry and clear, with highs below 40 and lows below freezing. Sunny on Thanksgiving. “There will be a real freeze,” Mass said.
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.