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Posted at 12:33 PM ET, 11/15/2011

Arctic air building in Alaska and Canada, will it stay there?


Surface temperatures (°F) this morning. Image courtesy of Unisys.

A core of exceptionally cold air has recently settled into parts of northwestern Canada and Alaska. The map of temperatures this morning across northern North America (above) indicates the severity of this arctic chill. Readings as low as 10 to 20°F below zero are common in the areas shaded in purple over western Alaska and the Yukon. Yikes.

And if that’s not frigid enough, it will likely get even a few degrees colder during the next couple of days in those very same places.

There are times when a temperature map like this one sends shivers down the spines of weather forecasters in the Lower 48. This is because in theory, it doesn’t require an unusual set of circumstances to drive that polar air into the United States. And in fact, we’ve seen it happen many times in recent winters. But this time around, the global weather pattern is in a shape that will likely keep most of that Arctic air up there for a while.


A 6-day forecast of high-altitude temperatures across North America. Image courtesy NOAA/ESRL
During the next week, we will once again see the emergence of a jet stream that will largely keep the southeastern half of the U.S. relatively warm (in the week-long average). This pattern is a regular guest during La Nina (which we are in right now) and consists (as shown to the right) of an upper-level trough (dip in the jet stream) in the West, a ridge in the East, and a strong southwesterly jet stream in between (outlined in black).

Though the image to the right is a forecast of the temperature pattern we expect to see next Sunday, where warm air sits firmly across the Southeast and colder air resides over much of the western North America, including the Northern Plains, it also happens to highlight some of the weather anomalies we saw just last weekend … which brought 70s to places like Kansas City, St. Louis, Nashville, Atlanta, and even Chicago. Given that, I wouldn’t be surprised to see similarly mild conditions once again surge into those same places by Saturday and to much of the East Coast on Sunday.

There is, however, a subtle distinction between these two versions of the western trough/eastern ridge theme. Unlike last time, the repeat performance upcoming in a few days will tap some of the Arctic air that is right now lurking at the top left corner of the North American temperature map, and allow it to leak southward into the Plains underneath the southwesterly flow aloft. The southward progression of Arctic air will be aided by north winds around a low pressure system that is expected to move across the Northern Plains early in the weekend.


Sea level pressure, thickness, and precipitation forecast (GFS model) for Saturday evening from NOAA/NCEP.
And though this system will only temporarily dent the otherwise inflexible southwesterly jet, it will likely give a gnarly little snowstorm on Saturday to those living in a narrow swath north of the surface low’s track. Though still several days out (with plenty of room for error), one reasonable outcome in this scenario is given by the most recent run of the GFS weather model (shown to the right), where the green shading over Minnesota represents a very cold and windy snowfall.

As that system moves eastward early next week into the Canadian Maritimes, it will leave a shallow cold airmass in place across the northern two thirds of the Plains and, at a lesser intensity, over the northern Ohio Valley and Northeast. Locations further south should be well protected from the chill by the larger pattern setup, which will continue to feature a southeastern ridge (of high pressure transporting warm air from subtropical latitudes).

As the end of the month nears, the resilience of this pattern, which tends to preserve unseasonable warmth in at least the southeastern half of the country, will shine. Even locales in the Northern Plains may warm up above normal by late next week.

One of the reasons why the Arctic air, currently in its loaded and ready position over the Yukon, will not punish the Lower 48 this time around is because there is no mechanism large enough to shove it deep into the United States.

Often times, the development of warm weather at polar latitudes can be sign that such a delivery system is in place. But we don’t see that right now.


Forecasts of high-altitude temperatures from the ECMWF (left) and GFS (right) weather models 6 to 10 days from now. Image courtesy of Pennsylvania State University.
And in fact, as shown to the right, 6-10 day forecasts of high-altitude temperatures offer little, if any, evidence that a large warm bubble of air (in orange/red shades) will occupy the northern territories in our sector of the globe anytime soon.

Instead, much of the wind at jet level in the next 1-2 weeks will have a west to east component (as shown by the straight black lines across North America) strong enough to prevent that from occurring, and therefore strong enough to keep the really cold air (shaded in blue) at bay for a while.

By  |  12:33 PM ET, 11/15/2011

Categories:  U.S. Weather, Latest

 
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