Confidence Building In A Major Arctic Outbreak

Well, another day has passed, and my confidence is building that we will see a major and prolonged arctic outbreak for the first half of January. We’ve got a lot to talk about, so let’s get right to it.

In my blog post last night, I talked about a massive ridge of high pressure centered near the International Date Line that would move east over the next couple days and eventually stall near 150 W as a very persistent “Rex Block.” Let’s take a look at how this ridge has progressed eastward over the past 24 hours.

Here was the setup over the Northern Pacific and Western North America last night at 7 pm. You can see the ridge centered over the International Date Line, a trough centered near 160 W, and strong zonal flow over the Pacific Northwest.

Now, here’s the setup 24 hours later. The ridge has “amplified” (strengthened and directed the jet stream further north) and moved east, now centered over 170 W. Additionally, the trough that was centered near 160 W has moved east to 150 W and “dug” southward. We currently have a weak ridge over the Pacific Northwest with a system tracking northward towards Vancouver Island.

By the way, this system will bring a fair amount of snow to the mountains, though nowhere near as much as we saw from Monday through Wednesday. Snow advisories are currently in effect from 4 am Thursday through 4 am Friday for 6-10 inches of snow in the Olympics and 11-17 inches of snow in the Cascades. Pass travel will be effected, so be prepared for delays and bring chains if you plan on traveling across the passes at this time.

As the next model frames show, the ridge will continue moving eastward throughout the next several days. At the same time, the deep trough to the east of the ridge will get “cut off” and eventually position itself directly to the south of the ridge. The model frames below show the evolution of this pattern in 12-hour intervals.

And by 7 pm Friday, the transition is complete.

As I explained in more detail in my previous post, this pattern with a high poleward of a low is a textbook example of a “Rex Block.” Rex Blocks are very stubborn and can remain over a given area in some form or another for over a week. This Rex Block is predicted to remain parked over the NE Pacific and send the jet stream WAY up north into Alaska, sending it southward along the Pacific Coast and bringing much colder air with arctic origins in the process.

The big pattern change that will begin our much colder regime begins Saturday night. A low pressure system will plunge southward, bringing a bit of moisture and ushering in a much colder air mass.

There is the potential for some snow with this feature, but amounts should be minor. As is often the case, the moisture will arrive while we are still too warm for snow, and by the time temperatures cool, we will be running out of moisture. There are some discrepancies in the models; the European model is colder, drier, and faster with this system while the American model (GFS) is warmer, wetter, and slower. At this point, it appears as though we will start out with light rain on New Years Eve and transition to light snow New Years Day, but specific timing and amounts are certain to change as we approach this event. It would certainly be a spectacle to have flakes flying as fireworks are firing, though.

The best way to judge the probabilities for lowland snow are to use ensemble forecasts. The Seattle National Weather Service forecast office created a handy graphic of the ensemble forecasts for snow at Quillayute, Bellingham, Hoquiam, and Seattle from this afternoon’s GEFS (GFS ensemble) runs. At this point, a few inches of snow look possible in the Western Washington lowlands, with less or a mix on the coast. But again, this is subject to change.

After our brush with lowland snow New Year’s Eve/Day, we turn dry and cold with sunny skies as arctic air surges down into the region.  There are some discrepancies in the models; the GFS is slightly warmer than the European model, but it is trending colder and the European model has been more consistent. As such, I am hedging my forecast towards the colder European solution.

The European model shows temperatures at the 850 mb level of the atmosphere of approximately -14 degrees C over our area Tuesday morning, which corresponds with a major arctic blast for the region. Monday will be cold with highs struggling to reach freezing, and Tuesday and Wednesday could see highs in the upper 20s and lows in the teens. This would be the coldest weather we’ve experienced in several years.

Looking at the extended forecast, we moderate a bit aloft Thursday through the weekend as the flow turns more northwesterly. At the surface, we will likely still be cold enough for snow, but moisture will be limited.

Although it is extremely far out, I’m VERY interested in what happens the week after next. Models hint at bringing a polar vortex (yes, THE polar vortex) somewhere close to our area. The most recent GFS stalls it to our north but gives us persistent onshore flow with multiple snowstorms.

It is dangerous (and honestly, a bit irresponsible) to talk about crazy model data this far out, but I just had to show this shot of -35 degree temperatures at 850 mb feet in southern BC. This is the stuff that makes weather geeks like me lose sleep at night. The chances of us seeing something this dramatic are very small, but the 10-15 day time frame could be very cold and snowy. Just a heads up.

Congratulations on making it to the end of this blog!
Charlie