Major Arctic Outbreak On Tap For Next Week

As a forecaster, it’s always a little nerve-wracking when models are consistently showing a high-impact event. In fact, in some ways, it’s more nerve-wracking than if the models were showing a variety of different outcomes. Higher model agreement gives us higher confidence in our forecasts, meaning that when that time inevitably comes that our forecasts are wrong, we fall flat on our face.

We’ve fallen flat on our face several times over the past couple years. There was the “Ides of October” storm on October 15, which was forecast to be one of the strongest windstorms in Pacific Northwest history but ended up being nothing more than a blustery day for most, and my friend Brie Hawkins of Little Bear Creek Weather also brought to my attention a massive windstorm forecast failure in December 2015. And I remember countless snow forecast failures as a child – including one in December 2008 where they canceled school due to an “uncertain forecast.”

But I have to say – I am shocked by how consistent models have been over the past several days in showing a major, extended arctic outbreak to start 2017, with the potential for coldest temperatures in years and a significant amount of lowland snow to boot. 

Let’s start by looking at the our current pattern. We currently have strong zonal flow (meaning the jet stream is oriented east/west with the lack of any significant ridges/troughs) in the wake of a cold front that came through the area last night. There is a large ridge over the Central Pacific that is pushing the jet stream far north into the Aleutian Islands. This ridge will play a tremendous role in our weather for the first week of 2017 and possibly beyond.

Here are the 500 mb heights (same thing as the green lines above) as modeled by the WRF-GFS at the University of Washington. The ridge is mostly off the screen at this time.

36 hours later on Thursday morning, the ridge has moved eastward, with a large trough off our coast and a ridge over our area.

Finally, by 7 AM New Years Day (72 hours after the previous frame), the ridge has moved even further east, with a deep trough right off our coast and a stubborn “cut-off” low to the south of the ridge.

The above configuration is a textbook example of a “Rex Block,” which is a blocking pattern characterized by a poleward area of high pressure and an equatorward area of low pressure roughly oriented north-south. This type of pattern is very stable and does not tend to move much. If the high associated with the Rex Block was further east and over our area, we’d see an extremely strong temperature inversion with fog and highs in the 30s at the surface with freezing levels near 10,000 feet. But with the Rex Block offshore, we get a large trough over our region, opening up the door for arctic air to plunge southward into the Pacific Northwest.

There is a chance of lowland snowfall on New Years Day as a low pressure system drops down from the north. But the real story with this low is that it will usher in a much, much colder air mass.

Models diverge after this period, with some (like the GFS) giving us a cold but not super cold arctic outbreak, with highs in the 30s and lows in the 20s. Others, like the European and Canadian models, give us highs in the 20s and lows in the teens, with possible single digits for outlying areas and locations away from the sound.

This far in advance, it is prudent to make use of ensembles, which are a suite of forecasts run from one model with slight purturbations in the intial conditions. The GFS ensembles below show the 850 mb (approximately 5,000 feet) temperatures over Seattle. They dip dramatically after January 1st , reaching -10 degrees C (14 degrees F) by the 3rd. As you can see, many ensemble members are cold for the entire run. Even if this arctic blast is not the coldest we’ve ever seen, it looks like it will be very long-lived. You can thank the Rex Block for that.

Unfortunately, I cannot share the ECMWF ensembles here (they are not public domain and therefore not fair use), but it’s probably for the best. We don’t want to get TOO excited.

I’ll have much more information in the days ahead.

Charlie