How long might widespread Arctic air stay away from the eastern half of the US this winter?
Here is what a polar vortex is in simple terms.
Anticipated changes in the upper atmosphere over the North Pole during late December may impact the polar vortex, which may trigger outbreaks of Arctic air over parts of the Northern Hemisphere during January and February.
"The AccuWeather long-range team has been closely following activity in the stratosphere and believes that an impending, significant disruption could have a major impact on the weather pattern after the calendar turns to 2019," according to AccuWeather Long-Range Meteorologist Max Vido.
The stratosphere is the part of the atmosphere that ranges from 6 to 32 miles above the ground.
Sudden stratospheric warming events (SSWs) tend to weaken the polar vortex.
"An SSW event occurs when temperatures in the stratosphere become abnormally high over polar regions," AccuWeather Meteorologist Kyle Elliott said.
As the polar vortex becomes distorted, the jet stream tends to buckle with large southward dips and northward bulges.
The northward bulges may send above-freezing air toward the North Pole. At the same time, Arctic air is then pushed southward as lobes of the polar vortex develop farther south.
The Arctic outbreaks may reach central and southern Europe, central Asia and the United States, but not usually all three at the same time. Sometimes the arctic air outbreak takes aim at Europe or Asia instead of the U.S.
"There were some minor disruptions in the stratosphere during the fall, but it is more common for major disruptions to occur during the winter," Vido said.
"Every SSW event unfolds differently in that there is not always an immediate response in the lower levels of the atmosphere," Vido said.
Sometimes, there is no way to say for sure which continent(s) or part of a continent will be affected until the outbreak is on the verge of occurring.
"When a strong SSW occurs, the Arctic air may be unleashed quickly or take up to several weeks to have an influence on the U.S. weather pattern," Vido said.
"Once the arctic air arrives, the colder and sometimes stormier pattern can last four to eight weeks," Vido said.
What does AccuWeather expect?
In terms of what is anticipated for the rest of December, no far-reaching blast of Arctic air is foreseen in the eastern U.S.
"We expect some significant cold to build over the northern Rockies and High Plains during the last week of the year, but only bits and pieces of that cold will drift eastward," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.
"Based on what we expect with all of the atmospheric conditions, the rest of December 2018 will feature some typical cold weather behind passing storms in the eastern part of the nation. However, these cold shots will be brief," Pastelok said.
January and February often bring the coldest weather of the year due to the lack of sunlight and water temperatures dipping to their lowest levels of the year.
However, at least parts of January and February 2019 may bring significant cold.
AccuWeather's long-range team has indicated since late this past summer that a pattern favoring lower-than-average temperatures and stormy conditions is likely to develop in a large part of the eastern half of the nation during January 2019 and that this pattern is likely to persist through much of February.
"Cold air will pour into the Northeast and interior South at times, and that is likely to lead to high energy demand during the heart of the winter," Pastelok said.
The same pattern that allows southward discharges of Arctic air to take place often leads to more extremes in the weather, including powerful, slow-moving storms.
"January and February are likely to be stormy with disruptions to daily activities, school closures and frequent travel issues, especially for the Eastern and South Central U.S.," Pastelok said.
Areas outside of the Arctic outbreaks, such as much of the western U.S., may be much warmer than average in the coming months.
There are other factors in addition to SSW activity that AccuWeather considers.
It is possible that these other factors may mitigate the magnitude and duration of the cold during January and February, even with a strong SSW signal ahead in late December and/or early January.
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