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 will stay mostly dry over the weekend. Expect uneventful weather tomorrow with clouds in the afternoon and then mostly cloudy skies on Sunday. There is only the option of sprinkling or showering on Wednesday. Enjoy your cool pre-Christmas weekend! My gut feeling at this moment? We will see some weather issues in some spots next week and possibly an ice/snow storm in others.

highlights

Uneventful weather is forecast for lowlands in western Oregon and southwest Washington through Monday (during the day). Lots of grey, a little cooler than normal and a shower or two.
Keep an eye on the forecast for Monday night through Wednesday morning. This is the first chance to stick snow at lower altitudes during this time. The same is also true of the northern Oregon coast and the southern Washington coast. Confidence in development (or will it just be a mix of rain and snow?) is pretty low right now. This is the first chance that winter weather can affect your plans. That said At this time we do not see widespread snowfall in the lowlands.
After mostly dry weather on Wednesday, a much wetter system may disperse moisture over the cold air flowing through the Columbia River Gorge on Thursday and/or Friday. That’s the second big possibility I’m watching. I’m confident that a snow/ice storm could be on the horizon in the canyon in the second half of next week. But could it be cold enough to cover at least part of the subway area with freezing rain or an ice storm? That would only be 2-3 days before Christmas. We will see…
There is strong agreement across all of our weather forecast models that the entire region will be milder towards and after the Christmas weekend (24/25).

Right now our 7 day forecast looks like this; If you have travel plans through the gorge in the second half of next week, that MIGHT be a problem. We are declaring these FIRST WEATHER WARNING DAYS to get your attention. For now, our temperature forecast is relatively conservative; There’s plenty of time for those temps to drop as models come together better over the next few days.

DETAILS

The upper level general pattern forecast has Not changed in the last few days. The great ridge blocking storms is moving westward, farther from us, this evening. This will allow cold arctic air to flow in from the Northwest Territories down into southern British Columbia for the next two days. Here is a map showing the flow at 18,000 feet (500MB) as streamlines, then 850MB temps are the colored areas; The blue is REALLY cold, like zero temperatures at ground level. this is sunday

This is how maps have been looking for a few days. At this point we get a few light rain showers but most of the action takes place north at the chasm. Now look at Tuesday; about the same on Sunday, right? The really cold air has stalled along the border

If anything, the atmosphere over Oregon is only slightly WARMER compared to late Sunday. At this point we have these 850mb temperatures around -5 which would only make snow fall at sea level when the wind is light and we get a flow of colder surface air from the north. That’s why the possibility of snow for Tuesday is a bit “marginal”.

Now here’s the change that’s been evident in the last 24 hours…it’s small, but it makes a big difference. This is Wednesday. The core of the Arctic air mass and the upper low move southeast, just along the Rocky Mountains. A “shortwave” trough or disturbance moves around the southwest side of the upper level low. Basically, it’s a “snipping” at the Pacific Northwest as it passes.

As of Thursday, some of that really cold air has made its way as far east as WA/OR right up to the Cascades. But of course, the core of the cold air moves straight south, right into the middle of the United States. We’ll hear all about it next week as it’s a pre-Christmas blizzard in the eastern US. Do you have plans for air travel east later next week? Much luck!

If this scenario occurs, it will result in quite a snow or (likely) ice storm near/in the Columbia River Gorge. It could even extend well into the I-5 corridor if The air is cold enough. That’s a big unknown 5-6 days away. That would happen when the Pacific humidity returns with a few wet weather systems Thursday and Friday.

So… that’s the note because there’s still quite a way out. Just stay tuned for the forecast when we head out early next week. I don’t have any vacation time scheduled until the Christmas weekend, so I’ll be keeping an eye on the cards/models very closely!