The Storm tonight and Sunday

This is going to be a complex event and I am going to take some time to describe it...and Sunday may end up different than some of the media is describing. I think we can have substantial confidence what will happen through 4 AM tomorrow...after that there is more uncertainty.

Before first, I wanted to comment on the remarkable temperatures we are seeing in western Washington. This morning much of the area has had temperatures in the teens (16F at my house in north Seattle), with many getting into the single digits. One location Arlington got below zero (I can hardly believe it). Here are some examples:

Univ of Washington 19

Mountlake Terrace 13

Lake Forest Park 9

Woodinville 8

S Everett 8

S Whidbey Island 8

Marysville 3

Arlington AP -3

Look at the plot of Seattle temps against normal. Our high temps are less than the normal lows. This is certainly one of the most sustained cold periods in recent memory (please no comments on global warming!--not an issue with a short local situation like this).Another remarkable feature has been the extreme snow in eastern Washington...with records at Spokane and nearby sites.

Now to the forecast. As you can see in the infrared satellite picture at 9:30 AM, the cloud band of with approaching system has reached our coast. The computer models are all in agreement that snow showers will begin in the early afternoon on the coast and move inland, with snow reaching central Puget Sound between 4 and 7 PM. It will snow this evening around here...this is very certain. But before we talk about snow, how about wind?
The high resolution computer models are going for strong winds developing this afternoon on the western slopes of the Cascades, the Columbia Gorge, and the Strait of Juan de Fuca. I have attached some of this output for you to see (for 4 PM, 10 PM today, 4 AM tomorrow). The predicted easterly winds along the western Cascade foothills are some of the strongest I have seen, with sustained winds of 50-60 mph with gusts that could reach 70-80 mph. I suspect the model is overdoing this....excessive downslope winds is a frequent model error and looking at its predictions for right now...the observations are less. This will be a very major event...but I suspect sustained 35-45 mph and gusts to 60-70 mph will end up closer to the truth. Very strong easterly winds (sustained 40- 50 mph) will also occur in the Strait of Juan de Fuca. There will be power outages.

Next the snow. As noted above, there is little doubt this Pacific system will make landfall and snow will start in the Puget Sound lowlands sometime later in the afternoon or early evening. There is also little question there will be a large east-west snow gradient over lowlands with heavy snow in the Kitsap and eastern slopes of the Olympics (1-1.5 ft), moderate snow near the Sound (3-8 inches), and lighter amounts towards the western foothills (1-3 inches). As noted in yesterday's blog this difference is due to the easterly flow...the resulting downslope flow over the western Cascade foothills produces drying and upslope on the Olympics causing increased precip on its upstream side. Attached is the 24h snowfall ending 4 AM. You see this features and others...such as the heavy snow in the mountains...and substantial snow over the eastern Strait. There is also a rainshadow (or snowshadow) north of the Olympics. Since I think the easterly downslope flow is being overdone a bit...as is the drying effects, I suspect that the snow amounts over Seattle and the east side are being underdone in the model, and this could increase the snowfall by a few inches.
So here we are around 4 AM with lots of snow. The cold front associated with this system will move through in the morning and there will be increased flow from off the ocean...and as a result there will be some warming aloft. A secondary trough of low pressure moves through during the afternoon on Sunday with more showers. These will bring snow in the mountains, but what about the lowlands? That is where this gets complicated and more uncertain.
Looking at the temperature predictions it appears the central Puget Sound and NW Washington could stay as snow...albeit wet snow and we could end up with several more inches. The coast will switch to rain as will parts of SW Washington. I don't see a major freezing rain threat in the PS lowlands.
But let me be clear...I am less sure about the second half of the forecast than the first part.
Anyway, be prepared to see a notable weather show.