Weather questions answered: What's the deal with this looming cold snap?

Forecast chart shows expected temperatures in the teens on Tuesday morning in the Puget Sound region. (Courtesy: WeatherBell)

We've already dealt with a few bouts of cold air this winter, but signs are pointing to Mother Nature upping the ante a notch as we head into 2017 with what appears to be a cold snap that could last for several days, possibly even a couple weeks!

A trough of low pressure will swing through Saturday night as we say goodbye to 2016 and hello to 2017, ushering in cooler air behind it. It'll bring a chance of lowland snow showers Saturday night above a few hundred feet, and down to sea level on Sunday. Snow amounts aren't expected to be that much -- about Trace-2" as a general rule with most areas around an inch or less. Then begins the days-long cold snap.

How do we stay so cold for so long?

We're getting into a classic cold weather pattern for the Northwest where a stubborn ridge of high pressure will become anchored Gulf of Alaska. That pattern takes the jet stream out of the Northern Pacific, shoves it waaaaay north into the arctic regions of Alaska, where it picks up quite a bit of cold air, then shoves that air down the backside of the ridge through British Columbia and into the Pacific Northwest:

Usually, the ridge hangs out there a few days and then moves on, the cold air "conveyor belt" collapses and we move on back to rain at times. But this time around, that ridge is expected to just park there for days and even when it finally gives way, more are waiting to take its place.

I heard it could be like December 2008 all over again?

That chatter came from a note inside the National Weather Service's forecast discussion on Tuesday where the lead forecaster noted in the extended range forecast: "This is starting to look like it could be the start of quite a significant winter weather event for the Pacific Northwest starting late New Years Eve or on New Years Day (one that has not been seen since December 2008)."

For those who weren't here (or who have chosen to block it from memory), December 2008 was a two-week arctic blast that featured multiple snow events in the Seattle/Western Washington area, including one nearly classified as a blizzard in the foothills, with a few sunny and windy but super cold days mixed in.

Here is a list of web stories/weather blogs I wrote during the two week encounter:

BUT the important part of that NWS forecaster's note was: "one that has not been seen *since* December 2008". December 2008 was the last time we had a multiple week event, but he's not saying it'll mimic the 2008 event. For one, this year's event is quite dry for several days to start. 2008's event was snowy from the get-go.

Then, how bad could this event be?

While we have forecast models that go out as far as 16 days, which can be useful in getting a sense of an overall pattern, going beyond a week is dicey as far as trying to nail down specific storms and such. We are moderately confident it will snow in spots Saturday night into Sunday, but no large accumulations are expected at this time. We have high confidence that it will be cold but dry through at least next Thursday with very hard freezes overnight (teens to low 20s) with daytime highs struggling to reach freezing.

Beyond Thursday, the extended forecasts have been surprisingly consistent that it will likely remain cold into that weekend and possibly into and though the following week. But it's too early to have any confidence to say anything specific like "heavy snow on Jan. 10th". There have been some pretty amazing/incredible wintry scenarios painted with each forecast model run, some suggesting widespread heavy snows, some more of a snow to freezing rain to rain, others a heavy snow-to-rain, others still just days with scattered light snow showers and yes, some with just enough warming to push it back to fringey rain/snow events. But none of them have been consistent to date, time, amount, or how they'd even play out. Right now, we're just wide-eyed as if we're watching a slot machine spin.

But just know the door appears to be open for... something. We saw earlier this month a cold stretch doesn't necessarily mean we'll get a lot of snow -- somehow Seattle missed out on much of it. Yet this time the cold air appears to be around a lot longer, increasing our chances.

Are *all* the forecast charts on board?

Most, but not all… (56 degrees, really?)

For the record, the Weather Channel has a high/low of 38/31 with rain & snow showers for Jan. 9th in Seattle.

I saw on Twitter people posting maps from forecast models that suggest lows in single digits in Seattle in two weeks. Really?

Another reason forecasts don't do so well beyond 7 days is the extended forecasts run in a lower map resolution after the 7th day. That's because it takes massive computer power to crunch the data and once the computers are done with 7 days, we turn down the computations needed in an effort to speed up the calculations so the model can finish before it's outdated. In the Northwest, it means the models then stop doing a good job accounting for the Canadian Coastal and Rocky Mountains.

So when arctic air spills down from Alaska, the long range models will think the air has free reign to just blow unabated right into the Pacific Northwest, when in real life, the Rockies will block much of that air and shuttle it east into Montana and North Dakota. Then the Cascades provide a second barrier keeping what arctic made it past the the Rockies mostly bottled up in the B.C. Interior and Eastern Washington, and only what can squeeze through the Fraser River Valley and Cascade mountain pass gaps is what gets into Western Washington.

Bottom line: If you see a low of 6 in Seattle on Day 10 -- that kind of cold air will likely be ticketed for places much farther east. Low to mid teens is about as cold as it gets in Seattle these days; it's been decades since Seattle has reached a single digit temperature. It's good to keep in the back of your head if you go out into the world of peeking at online long range models that a lot of arctic outbreaks in Day 8-15 that appear to make it into our region will take a "right turn" and end up farther east.

(New super computers just purchased by NOAA are expected to allow us to soon run the extended maps for a longer period in higher resolution mode.)

Speaking of "right turn", could this whole thing bust like the windstorm?

Sure. We're still predicting the future. But our forecast tools have been very consistent at least about the overall cold pattern, giving confidence to that set up. (And yes, it'd now be nice if it stayed cold to ice down that salt you just poured into that open wound.)

If it stays cold for that long, could lakes freeze over?

Fresh water can begin to freeze at 32 degrees, so it's easier to get ice going there. Green Lake has frozen over several times in Seattle's history (although not very frequently in years' past.) Lake Union and Lake Washington can also at least get pockets of ice with an extended freeze, and we have some concrete evidence that Lake Sammamish and Gig Harbor have frozen solid, although that took a historical 6 week cold snap in January/February 1950 that has yet to be even close to equaled since.

Take a look at this home video of ice skating on Gig Harbor in early 1950:

And here's a photo from Lake Sammamish, also in Feb. 1950, courtesy of the Sammamish Heritage Society:

As far as Puget Sound goes, that is salt water, and that has a lower freezing temperature. Seawater freezes somewhere around 28 degrees instead of 32, so it's more difficult to get ice there. Not impossible, I figure it'd be quite rare.

Now, various ponds and small streams and lakes could freeze over. But just don't find yourself on thin ice. Remember, it's rare to have it cold enough long enough for ice be thick enough to support a lot of weight.

Could it set records?

If January 1950 hadn't happened…maybe. But most of our January records are now in single digits thanks to that epic winter that, as mentioned froze everything around. Lows this time around would likely bottom out in the mid-upper teens in Seattle.

But this cold stretch could snap an impressive streak. It's been over four years since Seattle has had back-to-back months considered colder than normal (June-July, 2012). December qualified -- ironically coming in just slightly warmer than December 2008 only with gobs less snow and winter mayhem. January looks well on its way to being colder than normal too, barring some radical shift to relentless warm rains at the end of the month.

Can we blame La Nina (pretty please?)

In the general sense, yes. La Nina years typically have frequent occurrences of similar chilly patterns. You've been hearing us bang the drum for months that La Nina winters are typically cooler and wetter than normal with better chances of snow. Well, here's your proof. Although if you want to directly blame something, blame Alaska since that's whose cold air we're stealing from.

I just moved here within the past 4 years, are you sure it really snows in Seattle?

Seattle has not seen a major widespread several-inches-of-snow winter event since January 2012. That storm had heavy snows, then a prolonged period of freezing rain that hit King and Pierce Counties especially hard (with heavier snows to the north). Snowstorms had been a bit more common before then -- usually we'd get 1-2 a year which means we are overdue now. You'll find that the city faces numerous challenges even in just a few inches of snow: Mainly very hilly terrain that makes it difficult to get traction on snowy roads, a lack of plentiful snow plows -- because they collect dust most of the time anyway -- and an extreme case of driver inexperience. Or inexperienced drivers that are unpredictable to drivers who are more experienced.

Most of the freeways will be in better shape, relatively speaking, but side roads may go unplowed or untreated.

When will the cold snap end?

So far, the end is not in sight. The end of the two week forecast charts still indicate a generally cold pattern. However, we're pretty sure it'd have to end sometime before July 5th.

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