Brutal Cold To End 2021, White Christmas Possible

The Pacific Northwest has seen tons of mountain snow over the past 10 days. Since December 9, Snoqualmie Pass has seen approximately 8 feet of snow, and all of the major Cascade ski resorts, from Mt. Baker in the North Cascades to Mt. Bachelor in Central Oregon, opened for limited operations last week. I haven’t made it on a ski hill yet, but I did go snowshoeing yesterday near Oakes Peak and hope to go on some more snowshoeing/skiing adventures soon.

Some folks in the lowlands have gotten a taste of snow as well. Portland saw some snow on December 18th as a strong cold front came through and the combination of heavy precipitation and cold air advection helped cool the atmosphere to freezing.

This monster cold front is changing rain over to snow. Big old flakes over Portland right now! Expect more slop on roadways in the hills that will be slippery. ⚠️ #PDX https://t.co/f4dpWnjONp

— Kelley Bayern (@KelleyKOIN) December 19, 2021

This was a special type of cold front known as an “anafront,” which has substantial precipitation in the cold air behind the front. The precipitation behind the cold front caused a substantial amount of dynamic cooling, which is where the temperature cools due to snow melting to rain, as the change of phase requires thermal energy. And on the night of December 18th, the post-frontal air was cold enough and dynamic cooling substantial enough to bring light snow to the higher elevations of the Portland metro area.

But it is looking increasingly likely that a much more prolonged period of cold – with possible periods of lowland snow – may be the cards for much of Western Washington and the Willamette Valley during the last week of December. Allow me to explain.

Highlights:

• A very strong ridge currently building over Alaska will cause a deep trough to form downstream of this ridge by Thursday. This trough will bring us cooler-than-average temperatures and, more importantly, open up the door to arctic intrusions from Canada.
• On Christmas Day (Saturday), a strong arctic airmass is expected to approach the Pacific Northwest, with arctic air reaching the Pacific Northwest later Christmas Day or early on the 26th.
• There is the chance of some lowland snow late Christmas Eve/Christmas for Seattle, and on Christmas Day or the 26th for Portland, as the arctic front marking the leading edge of this first arctic airmass moves south. We should have enough precipitation, but temperatures may be on the fringe for snow in the major metropolitan areas. It’s a low confidence forecast at this time.
• After the 26th, the Pacific Northwest will likely experience an extended period of very cold temperatures that could last all the way through the first week of January. It’s too early to pin down any individual snow events at this time, but any precipitation that does fall over the area would likely be in the form of snow.

Forecast:

So far this month, the jet stream has been in a relatively “zonal” pattern across the Contiguous US. Flow has primarily been from the west to the east, and there haven’t been any notable intrusions of arctic air over the Lower 48 This has kept the infamous “polar vortex” up in northern Canada and, with the exception of cooler-than-average temperatures across the West Coast due to the troughing characteristic of La Nina, has resulted in much warmer-than-average temperatures across most of the US.

However, all of that is about to change this week. The big “blocking ridge” of high pressure in the North Pacific – visible as the red blob in the picture above – will strengthen and stretch all the way north into Alaska. This will cause a deep trough to form over the Pacific Northwest, opening up our region to a brutally cold arctic airmass.

The current 500mb heights show this ridge already extending north into Alaska, but while there is a deep trough offshore, there is not yet a clear path for arctic air to flow into the Pacific Northwest.

However, this Alaskan ridge will retrograde slightly to the west by Thursday morning, giving a clearer path for cold, continental, Canadian air to surge into the Pacific Northwest. Temperatures should drop into the lower 40s on Thursday with showers – potentially cool enough for some overnight snow on the higher hills outside the city, but nothing of note.

But by Christmas morning, our first shot of arctic air is right on our doorstep. And in the days following Christmas, this arctic air will be firmly over the area.

Let’s take a look at the ensemble models – models that contain a suite of many individual forecast “members.” The chart below shows the temperature forecasts from the European ensemble for Sea-Tac. I know it’s quite a busy chart, but the bottom line here is that there are a LOT of cold members – several with temperatures in the teens – and relatively few mild members. Additionally, note how many of the members keep arctic air over us through the first week of January.

Here’s a simplified chart, showing the predicted high/low temps from this morning’s ECMWF ensemble mean. Again, note how there is not only good ensemble agreement in extremely cold temps, but decent agreement in a very prolonged arctic outbreak that could last into the beginning of January.

White Christmas?

One of the tricky things about Pacific Northwest snow forecasting is that it is very hard to get both moisture and cold air in the same place at the same time. This is because our arctic air has cold, continental origins, while our storms arrive off the relatively mild Pacific. However, there are certain scenarios where we can get both, such as a strong Pacific storm making landfall just to the south of a region and sucking in cold, arctic air while spreading moisture overhead, or a strong arctic front bringing a quick shot of intense precipitation as it tracks south through the I-5 corridor.

We may see the latter on Christmas. We should have cool, NNW flow over our area on Christmas, but won’t yet have bona-fide arctic air in place. However, arctic air is expected to be in place by the 26th, and it appears that some precipitation may form ahead of this arctic airmass along an “arctic front” that will move from north to south across the Pacific Northwest late on the 24th or the 25th. The main question is if temperatures will be cold enough for snow, and that could be determined by many different factors – the intensity of precipitation, the exact timing of precipitation, temperatures ahead of the front, etc. Definitely something to keep your eyes on!

Stay warm,
Charlie