Snow showers continue over western Washington this afternoon as cool, moist air streams in off the Pacific. Portland and the northern Willamette valley are really getting hammered. Here's the latest composite radar image for the region.
Showers will continue tonight, but the big snow event won't be until late tonight and into tomorrow as a relatively weak surface low approaches the coast. Here's the 27-hour forecast from our 12Z initialized, 36 km WRF for tomorrow morning:
As the surface low moves in, there will be a strong pressure gradient across the mountains--high pressure associated with the cold air inland and the lower pressure associated with this warm frontal wave off the coast. This pressure gradient is going to draw down cold air near the surface out through gaps in the mountains and down into the lowlands. Meanwhile, the warmer, moist air being brought up from the south will tend to lift over the cold air. That lifting will cool the moist air, clouds will form, and precipitation will fall out into the cold air near the surface. Here's a diagram that hopefully will help explain that setup:
However, locating that boundary between the cold and warm air is critical for determining where the snow will fall. If the warm air moves too far north, Seattle will be too warm for snow and we'll get a lot of rain. If the cold air pushes too far south, the warmer, moist air will be precipitated out by the time it reaches Seattle and we'll see less snow. This is all related to the placement of the surface low. Here's two scenarios:
First, let's say the surface low stays further north--approaching the coast from the Strait of Juan de Fuca, perhaps. In this case, warm, moist air will be brought up into western Washington. However, because there is that nice gap between the Olympics and Vancouver Island (the Strait of Juan de Fuca), the surface low will draw most of the cold air straight west and very little will filter south into the Seattle area. This would turn into an all rain event for Seattle. I show this setup below.
And it looks like our models so far are favoring a southern route. Here's last night's 00Z forecast from our 4km WRF for where the surface low would be at 18Z on Wednesday (Wednesday morning).
But, wait...this was last night's model. What about this morning's 12Z run?
This paints a slightly different story. Here's this morning's model surface forecast for 18Z Wednesday (the same time as the surface map above):
This change in precipitation amounts is very sensitive to the exact location and timing of the arrival of warm air and the low pressure center on the coast. One thing we do know is that with this kind of model trend (moving the colder air further south), it should definitely be cold enough to support snow through the duration of this event. We're not as concerned about the transition to rain at the end anymore. The big question is still what final precipitation amounts will be. The low may end up moving slightly further north and increase the snow amounts again--we'll have to wait and see what tonight's round of models have in store.
Even with the slightly diminished amounts, we're still looking at a major snow event late tonight and tomorrow in the Seattle area. It looks like the only questions now are in the details.