Watching Tuesday through Thursday closely! Cold arctic air could cause trouble in parts of Pacific Northwest

Today was a perfect cool December day, and we’ll stay mainly dry through the weekend. Expect uneventful weather with afternoon clouds tomorrow and then mainly cloudy skies Sunday. There’s just a chance for a sprinkle or shower Wednesday. Enjoy your cool pre-Christmas weekend! My gut feeling at this moment? We’ll see some weather issues next week in some spots and possibly an ice/snow storm in others.

Highlights

Uneventful weather is likely through Monday (daytime) in the lowlands of western Oregon and SW Washington. Lots of gray, a bit cooler than normal, and a shower or two.
Keep a very close eye on the forecast for Monday night through early Wednesday. That’s the first chance for sticking snow to lower elevations during this time. Same thing out on the northern Oregon and southern Washington coastline too. Confidence in how that evolves (or will it just be rain/snow mix?) is quite low right now. That’s the first chance of winter weather capable of affecting your plans. That said, at this point we don’t see a widespread snowfall in the lowlands.
After mainly dry weather Wednesday, a much wetter system may spread moisture over the top of cold air pouring through the Columbia River Gorge Thursday and/or Friday. This is the 2nd big possibility I’m watching. I’m feeling confident a snow/ice storm could be on the way the 2nd half of next week in the Gorge. But could it be cold enough to bring freezing rain or an ice storm to at least part of the metro area? That would be just 2-3 days before Christmas. We will see...
There IS great agreement among all our weather forecast models that the entire region turns milder heading INTO and beyond Christmas weekend (24th/25th)

For now, our 7 Day forecast looks like this; if you have travel plans through the Gorge 2nd half of next week, that MIGHT be a problem. We’re calling those FIRST ALERT WEATHER DAYS to draw your attention. Our temperature forecast is relatively conservative for now; there’s plenty of time to lower those temps if models come together better the next few days.

DETAILS

The general upper-level pattern forecast has not changed in the past couple of days. The big ridge blocking storms is moving westward, farther away from us this evening. That allows a plunge of cold arctic air from Northwest Territories all the way down to southern British Columbia the next two days. Here’s a map showing flow at 18,000′ (500mb) as streamlines, then 850mb temps are the colored areas; the blue is REALLY cold, like temps down around zero at ground level. This is Sunday

This is what maps have been looking like for several days. We’re getting a few light showers at this point, but most action is up north precip-wise. Now look at Tuesday; about the same at Sunday isn’t it? The really cold air has stalled along the border

If anything, the atmosphere overhead is just slightly WARMER over Oregon compared to late Sunday. At this point we’ve got those 850mb temps around -5, which would only drop snow to sea level if wind is light and we’re getting a drift of colder air at the surface from up north. That’s why the snow possibility for Tuesday is somewhat “marginal”.

Now here’s the change that has shown up the past 24 hours...it’s minor, but makes a big difference. This is Wednesday. The core of the arctic airmass and upper-level low is making a move to the southeast, right along the Rockies. A “shortwave” trough, or disturbance is moving around the southwest side of the upper level low. Basically it’s “taking a swipe” at the Pacific Northwest as it goes by.

By Thursday a chunk of that really cold air has made it down into Eastern WA/OR right up against the Cascades. But of course the core of the cold air is heading straight south into the middle of the USA. We’re going to be hearing all about that next week since it’ll involve a pre-Christmas snowstorm in the eastern USA. If you have air travel plans back east later next week? Good luck!

If this setup occurs, then that leads to quite a snow or (more likely) ice storm near/in the Columbia River Gorge. It could even extend well out into the I-5 corridor if the air is cold enough. That’s a huge unknown 5-6 days out. That would happen as Pacific moisture returns with a couple wet weather systems Thursday and Friday.

So...that’s it for know since it’s still quite a ways out. Just stay on top of the forecast as we head into early next week. I don’t have any vacation time scheduled until Christmas weekend, so I’ll be keeping a very close eye on maps/models!