We’re very close to what could happen this week: close enough to deliver higher resolution samples in time.
As I will explain, the upcoming situation is likely lowland snow but not optimal. Heavy snowfall in the mountains is certain.
Snow Donkey
Let me remind you, ISnow is hard to come by in the lowlands west of the Cascades. Moist air from the Pacific Ocean is too warm for lowland snow, and it is difficult for cold continental air to reach us due to the blocking effects of the Rockies and Cascades.
So to get snow, we need something weird to happen. The most common trick is to get a low center off the south coast of Washington and a cold high pressure inside BC. The difference in pressure causes cold air to be sucked southwest through the Fraser River Valley or through lowland into Northwest Washington, then spread south. Lowlands on the Washington coast and an accompanying upper-level low-pressure gully lead to rising air that produces precipitation that falls into the cold air near the surface.
That can lead to snow.
Another way to ensure snow in the lowlands is to have cold air in place (from above), while a wind blows inland. Precipitation starts out as snow and then turns into rain.
We might get a bit of both this week.
Nowaday situation
Last night, a relatively strong cold front moved through and temperatures above (and near the surface) dropped significantly.
The cold air also produced a strong northwesterly wind in the Juan de Fuca Strait and downstream around Everet, generating gusts of up to 50-60 mph last night (see gust diagram below). Thousands of customers lost power
Tonight, a weak depression will move over western Washington and a cold, high pressure will form over British Columbia (see surface map forecast for 4 a.m. Monday). By the way, blue is cold enough for snow. Monday morning will bring a hard freeze in western Washington that will be at a 20-year low.
Cold, unstable air will move into the Cascades over the next day, with some snow accumulating in the mountains (see total snowfall through 4 p.m. Tuesday below)
Fourth situation
Big action is likely to happen on Wednesday. A strong upper trough will approach, generating precipitation over the area….where there will be cold air (upper floor map shown at 4 a.m. Wednesday)
A strong surface depression will approach north of Vancouver Island, with an associated front set to make landfall on the Washington coast Wednesday morning (Wednesday 1:00 a.m. surface map is shown). This front will eventually pull in warmer air, but precipitation could also start in the form of low-level snow Wednesday morning. A large amount of snow will fall in the mountains. I
I would like to stress that this is not the optimal situation for snow in western Western Australia. For a major snow event, you really want that low to be parked along the south coast of Western Australia, sucking in cold air from BC.
The UW’s latest high-resolution forecast for snowfall through 3 p.m. Wednesday is shown below. Many feet in the mountains, but note the snow band from Seattle to Everett (a few inches). Eastern Washington will also see snowfall. Of course, the depth of the snow will be less than the amount of snowfall.
The lower resolution Central European model has a similar solution (see total snowfall for the same time period..now until 4pm on Wednesday). Lots of snow in the mountains, snow in NE Washington, and a swath of snow in the lowlands from Seattle to Everett.
Our confidence in the Puget Sound snow forecast will increase as we get closer in time and with high-resolution ensembles available.
