Good news, winter weather lovers. Snow is returning to the Cascades, and it’s doing so in a big way.
Snow will start impacting the Olympics and Coast Mountains of British Columbia early Tuesday morning as a wet cold front swings through the area. Snow levels will hover around 3-4000 feet, but all the passes – including Snoqualmie Pass – should see snow in the evening as cool easterly flow will keep snow levels near the surface. Here in the lowlands, things will be wet and cool, with the potential for up to an inch of rain in Seattle, more as you go west.
Snow levels will plummet to 2,500 feet in the wake of Tuesday’s front as a cool upper-level trough moves into the region. The atmosphere will be unstable (large decrease in temperature with height), and showers should stream into the region throughout the day. The Olympics could see up to a foot of snow, but the passes should see lighter amounts near 4 inches, heavier further north. Expect showers in the Seattle metropolitan region throughout the day, with an increase in the afternoon/evening.
The chart above shows the “thicknesses” of the atmosphere between the 1000 and 500 mb levels of the atmosphere. Because thickness is directly proportional to temperature via the hypsometric equation, the lower the thickness, the cooler the average temperature of the layer. Generally speaking, thicknesses under 540 dm (decameters, or 10s of meters) are required for snow at Snoqualmie Pass, and thicknesses under 522 dm are required for sea-level-snow in the Puget Sound region. If you want a brief primer on the hypsometric equation and atmospheric thermodynamics, I encourage you to read a post I wrote back in 2013 here.
Serious snow arrives to the mountains for Thanksgiving as a strong cold front storms into the lowlands. From Thursday to Friday night, snow totals will be measured in feet, not inches. We’ll also see heavy rain in the lowlands, particularly for Portland where 1-2 inches may fall between Thursday and Friday morning. We’ll also be breezy with the chance for 50 mph gusts on the coast and 30-40 mph gusts inland. If you are planning on traveling across the pass for Thanksgiving, Wednesday will be best. Even then, bring chains and take it slow, as light snow will be falling for much of the day at all the passes.
Models have been inconsistent with the details, but the latest model runs have light snow for Friday and Saturday at the passes and dry us out for Saturday. Sunday night, another cold front swings in from the northwest and brings another round of heavy snow to the mountains. Most ski resorts will have enough snow to open by the end of next week.
Things get interesting in the far extended, as models have shown a strong ridge of high pressure developing near 140 degrees west. If the ridge was further west and extended further north, we could see a large trough over our area with the potential for lowland snow. At this point, lowland snow looks highly unlikely and I am fairly certain that the Western Washington lowlands won’t see any flakes for the next two weeks, but it’s always fun to see hints of winter in the models.
Charlie
