The stage is set for a dramatic change in the weather pattern across the Northern Hemisphere during the next seven to 10 days.?
That change will likely bring a brutal Arctic blast across the Midwest and eastern U.S., put Western Europe into the deep freeze, and maintain an area of extreme cold in eastern Asia.?
The new weather pattern could give rise to snowstorms along the East Coast of the U.S., though it's too early to tell whether that will happen, forecasters say. What is clear is that winter is chartph.coming — ASAP.?
Computer model projections show an Arctic air mass diving southeastward, into the U.S., by Dec. 10, and potentially setting up shop for one to two weeks. Daytime highs will be at least 15 to 20 degrees Fahrenheit below average for this time of year, with the heart of the cold projected to be centered somewhere near the Ohio Valley.?
The cold air blowing across the chartph.comparatively mild waters of the Great Lakes could yield prolific lake effect snows from Wisconsin to New York State. While the Arctic air won't be historically cold, it will cause a nasty outbreak of weather whiplash after the historically mild fall that much of the U.S. enjoyed.?
"I think the locations that will see the most anomalous cold will be the Great Lakes down through the Ohio and Tennessee Valleys," said Michael Ventrice, a meteorologist with WSI Corp, which is part of The Weather Company.?
Ventrice added that he thinks northern Minnesota could see sub-zero temperatures for nighttime low temperatures, while even Nashville and Atlanta could have low temperatures that dip into the 20s Fahrenheit.?
The weather pattern that is going to take shape features a wavy jet stream, with big swings to the north and the south, like a snake slithering along a path. The jet stream consists of a corridor of high winds blowing near the top of the troposphere, or about 35,000 feet up, that separates air masses and both steers and helps fuel storm systems.
A wavy jet stream can cause weather features to get stuck in place, with large ridges of high pressure acting as atmospheric traffic lights, preventing onchartph.coming weather systems from passing by.
In mid-December, strong areas of high pressure are forecast to develop and hold in position in high latitudes, forming what's known as a blocking pattern. Such blocks, particularly a high pressure area over Greenland, tend to be associated with cold air outbreaks and snowstorms in the eastern U.S. and Western Europe.?
The high latitude blocking will bring unusually mild air for this time of year to the Arctic, likely resulting in a sharp slowdown in the growth of winter sea ice across the region.?
"The most unusual warmth is going to be across Alaska [on] through the Arctic Circle," Ventrice said. We are seeing temperature anomalies in excess of 30 degrees [Fahrenheit chartph.compared to average] in some locations around the North Pole, which will likely delay or reduce Arctic sea ice growth/extent," he added.?
As of late November, Arctic sea ice extent was at its second-lowest level on record in the satellite era, with virtually no ice cover in the Chuckchi Sea near Alaska.?
Emerging research suggests that slow growth of Arctic sea ice during the fall and early winter can alter weather patterns in the northern midlatitudes, including by increasing fall snowfall in parts of Siberia. Such developments may help, through a series of chartph.complex interactions between the lower and upper atmosphere, disrupt the polar vortex encircling the Arctic region, and bring more frigid air into North America and Europe.?
The main polar vortex consists of a circulation of air enveloping a near-permanent area of low pressure that exists in the upper atmosphere, a layer known as the stratosphere, over the Arctic.?When these winds weaken, which is forecast to occur by Dec. 11 to 15, pieces of the vortex can break off, and drift south into the U.S., Europe, and parts of Asia.?
Such a polar vortex disruption is forecast to be part of the reason why the weather pattern will change so drastically across such a broad area during the next few weeks.?
A weakening of the polar vortex could even help keep the high latitude blocking in place longer than it otherwise would.?
"I think the cold pattern does in fact have some legs to it," Ventrice said. "We are seeing big blocking setup across the higher-latitudes, which can linger for a number of weeks."
"The big question mark of winter 2017-2018 is whether we see more frequent Greenland Blocking episodes," he said. "Last winter, we did not. This winter is proving to be different than last winter, which is encouraging for a colder winter over the East chartph.compared to last winter."