It’s been the buzz of Pacific Northwest weather geeks for several days now.  And at this point, models are zeroing in on the basic pattern.  So it’s time to say:

Our region is looking at a significant winter cold event, beginning during the weekend of December 25-26.  We don’t know yet just how cold it will get, but it will most likely be the coldest weather we have seen so far this season.

HIGHLIGHTS:

1.  Forget about lowland snow on Christmas Eve; the airmass is a few degrees too mild.  Even Christmas Day looks unlikely at this point unless we luck out with a heavy precip/dynamic cooling scenario.
2. Colder air moves in Sunday and Monday, which will bring snow levels down to the valley floors.  But it’s very tough to determine just how much snow falls at the lowest elevations.  Details on airmass temperature and moisture are always hard to pinpoint!
3. Tuesday and beyond next week is a big question mark.  We could see bitterly cold arctic air over the region, or just more chilly onshore flow with very low snow levels.  Or we could see a snowstorm with cold east wind spin up.

There…that was simple enough, wasn’t it?  Generally when it comes to cold and snowy weather, I’m hesitant to blog about it too far in advance.  Projected arctic blasts and snowstorms on the models are notorious for being fickle; and far too often what looked like a promising pattern 7 or 8 days out, becomes far less impressive as the event approaches.  The pastime is called “model riding” for a reason!  Thankfully we’re down to the 5-day window now, and there is excellent agreement on maps that something cold is going to happen early next week.  But the verdict is still out on the fine details (temperatures and snowfall).

Let’s look at the current 500mb anomaly maps, which give us a good picture of how the atmosphere and the jet stream are behaving.  First, North America & surrounding waters for tonight:

This shows cold anomalies out over the eastern Pacific, and a huge warm anomaly (blocking ridge) in the north central Pacific.  Blocking ridges like this, setting up near 160W, are common at the beginning of a cold episode.  In this case though the cold hasn’t arrived yet; there is too much energy off the coast of California in particular.  But look what happens by Tuesday afternoon next week:

The ridge has expanded further northward and eastward, and is nosing into the western Yukon!  Also notice ridging over the southeastern U.S.  These two features help “wedge” the cold anomalies into the Western U.S., and open the door for frigid arctic air to surge south.

But how far south it comes is where the models are still unclear.  Let’s look at tonight’s 00z maps for the 850mb airmass temps and winds.  First, Sunday morning 4pm, which is the morning after Christmas:

850mb temps of -7 over Portland…that means snow levels dropping to below 1,000 feet!  Whether it comes down to the lowest elevations, though, is highly contingent on how heavy the precip is.  But in this case, we can just wait 36 hours for colder air:

Now the airmass is down to -10C over Portland and Vancouver…brrr!  Any precip would definitely fall as snow, if there is still moisture around to work with.  But again, we don’t know the details 6 days out.  Now on to next Wednesday morning, the 29th:

Now THIS is interesting!  Low pressure to the south of Portland and arctic air in the Columbia Basin.  This is a classic recipe for a snowstorm with east wind.  But again…it’s way too early to start predicting where a given low is going to track.

But the ECMWF from earlier today has a different idea.  It brings a massive blast of arctic air to the entire Pacific Northwest.  The 850mb temps drop to -17 for Portland and about -23 for The Dalles.  That’s a top-tier cold blast!

So there is a lot of disagreement in terms of how far south the arctic front comes.  If the ECMWF wins the battle, we have a major deep freeze on the way next week with temps plunging to 10F or even colder west of the Cascades at night, and likely only around 20F during the daytime.  If the GFS is correct, the arctic air stays further north and east, perhaps briefly slipping into the Columbia Basin and Gorge.  That latter scenario could set up Portland for a snowstorm next week, if an incoming low pressure moves to our general south.  But temperatures would not be as cold in that scenario, especially not at night.

Even on the GFS there is significant disagreement among the various members of the forecasting “ensemble.”  Quite often in the long-range, the member used for map-making (the “operational”) will be much warmer or colder than the average of the other members.  But if the average, or ensemble mean, is well below normal at the 850mb level…it means that some kind of cold pattern is imminent.  Sure enough, the ensemble mean (white curve) on the 18z drops to -9C.  That isn’t quite arctic in and of itself, but it means that many of the colder members do bring in arctic air.

The ensemble for surface temps in Vancouver is telling.  The mean shows a low temperature of -10C, or 14F, on the morning of the 29th.  That’s cold!  Also high temps on the white squiggly line drop below freezing for a few days next week…then hover near freezing for the first few days of January.  This suggests the general cold pattern will hold on for quite a while beyond the holiday weekend:

I’m not going to post any snow total maps; that would be a fruitless thing to predict in this pattern.  The big message is that we have a shift to colder conditions beginning this Sunday or Monday.  It might just be moderately cold, with a few flirtations with lowland snow.  Or it could be a full-blown invasion of arctic air, with daytime temps way below freezing for several days.

Start making plans accordingly.  Make sure your home and car are properly winterized.  Protect any tender plants from the threat of a hard freeze.  Keep a close eye on the weather forecasts over the next few days as we refine the details of exactly what might happen.

Happy Holidays everyone…stay warm!

Karl