The science behind a wintry intrusion this week

Any time the pattern gets as anomalous as it is forecast to get over the next several days, a meteorologist’s job becomes very interesting. We can either simply say “It’ll be very cold this week”, or we can take the time to explain the science behind it. We’ll choose the latter. The pattern change, which will undoubtedly be noticeable by the middle to end of the upcoming work week, is occurring as a result of several impressive atmospheric events. Global circulations are changing the pattern — and it will be a sight to see. But the change goes far beyond the fact that unseasonably low heights will dip into our area by Thursday morning.

The pattern is already unbelievably anomalous well to our west, over the Northeast Pacific and British Columbia. The extratropical transition of Typhoon Nuri and its phase with a piece of Pacific Energy led to a tremendously anomalous trough — and an incredibly strong storm system with minimum central pressures down to 924mb (The strongest storm in the history of the Bering Sea and the North Pacific). But, more importantly, the anomalous storm system is amplifying a tremendous ridge to its east over the West Coast of the United States, northward into British Columbia and toward the poles.

At first, this may seem like something that isn’t necessarily too foreboding for our area. But when the exact progression of the pattern is exposed, the nature of the displacement of cold air can become fully understood. The GEFS ensemble members show the incredibly anomalous ridge developing from the West Coast through British Columbia into Western Canada by the middle of this week. Obviously, cold air that was bottled up to our north will be displaced southward through Canada toward the Northern 1/3 of the United States. This is, in fact, a response to a now plunging Arctic Oscillation and plummeting EPO (Eastern Pacific Oscillation).

GEFS mean height anomalies at 500mb for the middle of this upcoming week. Notice the highly anomalous ridge in Western Canada.

GEFS mean height anomalies at 500mb for the middle of this upcoming week. Notice the highly anomalous ridge in Western Canada.

Perhaps the biggest harbinger of the pattern change, still, isn’t the ridge in the Western part of Canada. It’s a feature that appears just a few days later — to make matters more anomalous than they already are. High latitude ridging, on all medium range forecast models, builds into Greenland and parts of the Davis Straight by the end of next week. What does this mean for us, you ask? It means that the Western Canadian ridge is causing a global response — and the high latitude ridging building and connecting to its east will assure that the cold air displacement over the United States isn’t “temporary”.

In fact, the high latitude ridging forecast to build over Greenland, through the Davis Straight, will create positive height anomalies at 500mb that reach all the way from the initial West Coast ridge response to Greenland. In essence, a classic high latitude blocking event. And this displaces cold air, typically still sitting over Northern Canada at this point in the season, thousands of miles to the south.

GEFS height anomalies at 500mb showing high latitude ridging and much below normal heights over the USA.

GEFS height anomalies at 500mb showing high latitude ridging and much below normal heights over the USA.

In terms of sensible weather, what’s occurring is a strong signal for below normal temperatures across much of the United States east of the Rockies — including our area. While, of course, there will be some airmass modification over time (we won’t exactly be hiding in our houses from the cold temperatures) the airmass will still be quite anomalous. Medium and long range forecast models are in good agreement on a prolonged below-normal temperature period through the next 14 days beginning at the end of this coming work week.

And what of snow, you ask? For that, we’ll have to wait and see. The pattern, as it stands now, will be supportive of frozen precipitation once the anomalously cold air enters our area. And the high latitude blocking means that, more than likely, we’ll have a few disturbances capable of producing snow in our area. Individual perturbations are extremely difficult to pin down at this point in time, but no — the talk of snow you’re hearing isn’t simply hype.

The cold air? It’s coming. And it’s likely here to stay.

Comments

comments