Weather Blog

Tackling the arctic air rumors for the end of November

Being a weather forecaster that is not on TV has its advantages sometimes -- namely that since no one ever recognizes you, it's easy to get an anonymous pulse on what city residents feel about the weather, while not knowing that you actually work in the field.

Between my wife and me, we've had four separate people (OK, mainly coffee stand baristas) comment on this rumor that it's supposed to get cold and/or snow next week and/or over Thanksgiving holiday.  The weather e-mail bin also has a few similar inquisitive people.

It's not like super-long range forecasting has gotten markedly better this past year -- no, a forecast 10-16 days out should rarely get any stock put in it. But these days with forecast models online and the lightning-quick ways information can spread via Twitter and Facebook, it's easier for information to get out there and among the masses. And that includes a few posts that begin around the weather geek crowd as "hey, check out this long range forecast that hints at cold weather" turn into "I hear there's a foot of snow coming".

(OK I admit I'm a bit guilty, as I jokingly Tweeted about what the 16th day forecast had in store for Thanksgiving. (The link in the Tweet to the forecast model has since updated so the map you see isn't what was there originally))

But there's a reason why mainstream weather forecasts wouldn't start banging this drum that early. For one, you all know we have enough of a challenge trying to forecast snow around here 24 hours (24 minutes?) in advance much less more than a week away.

Admittedly, the super-long range forecasts have been fairly consistent over the past three days in at least advertising a colder weather pattern starting around next Thursday and more or less hanging around through the Thanksgiving weekend.   The details shift wildly each forecast model run, as you would expect that far out, so it's pointless to try and nail down any real forecast, and even so, the colder air doesn't appear to be *quite* cold enough yet for winter's favorite four-letter "s" word. But the theme is there.

Now, when you look at these models, aside from the issues that arise from any error in its short term calculations getting greatly multiplied as the forecast moves farther along there is one other major factor to consider when you glance at a model and it shows cold air piling in from Canada:

The models don't have a very good clue about those big mountains to our east.

It takes a whole lot of computer power and time to crunch the mathematics needed to simulate the atmosphere. Once the computer has crunched 7 days' worth, it's getting kind of late and we're up against a deadline to get the data out to the public before it becomes outdated. So for days 8 and beyond, the models run in a quicker mode that uses less data points to hurry up and get those last 8 days computed -- mainly for forecasters' yucks. There is a reason we don't go beyond 7 days on TV.

(People who use these models will notice once you reach day 8, the predicted blobs of rain get a whole lot less precise. See how the one on the left has more descriptive green blobs than the Day 8 map on the right?)

For us, one critical trade-off in running in this "diet" mode is that the Cascade and Rocky Mountains are not accurately accounted for. So the model sees this arctic air in Alberta or B.C. and figures it'll just spill on down into Western Washington, but it's way overdone.

It's like bragging on Facebook you can run into your living room from the bedroom at full sprint but not factoring in there is a bedroom door... and it's closed.

In real life, since cold air is heavy and dense, the Rockies will block some of that cold air from getting west. Some arctic air can spill over into Eastern Washington, but then we have the Cascades that will block it from getting into Western Washington.

Rarely does arctic air just flow into our area -- it needs to have a mechanism to draw it out and the frequent culprit is a low pressure area moving in to the south of us. That low will act like the vacuum cleaner to draw the arctic air into our region through the gaps in the Cascades and Fraser River Valley.

But in most of the current long range models, we don't have the low (some have hinted, but not consistently).

Suffice to say, while the long range models do look kind of cold right now, it's probably overdone. We have to wait until the time gets closer so that A) the models factor in the mountains better (even better yet is when it gets within 72 hours and we have the higher resolution UW models to work with) and B) don't have as many calculation errors in them that have likely exaggerated over time.

I will say that there should be some high optimism for skiers that the mountains will start piling up some more snow by the end of the month. As for us in the lowlands? The long range weather is something neat to ponder over a cup of coffee, but there's no reason to freak out over it yet :)