Contrary to popular belief, that doesn’t mean widespread snow or cold for the Lower 48. In fact, the opposite may be true, with unseasonable warmth and mild temperatures more likely for most of the southern, central and eastern United States.
What is the polar vortex?
The polar vortex is a staple of the atmosphere; the southern hemisphere has one, too. Each polar vortex has two parts — the tropospheric polar vortex, which occupies the lowest level of the atmosphere in which we reside, and the stratospheric polar vortex up above. The tropospheric polar vortex is usually wavier and more erratic, while its counterpart in the stratosphere tends to be smoother and more self-contained.
Both are “coupled,” meaning changes in one can influence the other. That’s why meteorologists look at the health of the stratospheric polar vortex for longer-range indicators of how the weather people actually experience evolves. A stronger stratospheric polar vortex tends to fence in the cold, while a weaker one allows Arctic outbursts to visit the mid-latitudes.
How does the polar vortex affect us?
“The weather during a strong polar vortex is usually not as exciting as we get when we have a weak polar vortex,” explained Hannah Attard, an associate professor of atmospheric sciences at Embry-Riddle Aeronautical University in Daytona Beach, Fla.
“When you have a strong polar vortex, it’s really acting to bottle up all that cold air at the poles, so you won’t get those undulations at the mid-latitudes letting that cold air spill out,” said Attard.
That looks to be the case for the next several weeks, at the very least delaying the onset of winter’s mischief.
Given “the fact that the polar vortex has gotten so strong … I think this one will be associated with overall mild weather to kick off December,” said Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research. He says that a La Niña pattern taking shape over the eastern Pacific could also encourage the polar vortex to remain strong.
“I’m not seeing much evidence to break it down or get it wavy and exciting,” agreed Attard. “I think for now it’s pretty steady and will keep that cold air bottled up north.”
An El Niño pattern, on the other hand, would favor a more rapid breakdown of the polar vortex, allowing the collapse of prolonged bitter chill to extend into the Lower 48. That’s not the case this year.
“With El Niño … we get [disturbances] that flux heat into the upper atmosphere and break down the polar vortex,” said Attard. “With La Niña, you may not get that as much.”
A mild start to winter for most, but uncertainty thereafter
The National Weather Service’s Climate Prediction Center is forecasting odds of above-average temperatures for most of the southern, central and eastern United States through late February; that could suppress snow totals for many. Only the northern Plains, the Pacific Northwest and the Inside Passage of Alaska are favored to see colder-than-average conditions prevail.
Granted, that forecast takes into consideration a number of other factors; the polar vortex is tough to predict further out than two or three weeks into the future.
“Using the models, the GFS, the European … those are at the weekly time scales we’re forecasting,” said Attard. “We can only forecast the polar vortex out about as far as those models go.”
She said atmospheric scientists can aim for broader generalizations about the vortex later in the season, but it’s impossible to make a definitive call so early.
“It’s strong right now, so it’s likely going to remain strong,” said Attard.
Amy Butler, an atmospheric scientist with the National Oceanic and Atmospheric Administration, agrees but mentioned some midwinter uncertainty.
“Seasonal forecasts initialized in November seem to have some skill at least through January,” wrote Butler in an email. “This year, the seasonal forecasts initialized in November somewhat disagree on the strength of the polar vortex in January, then more consistently show a strong polar vortex in Feb/Mar.”
In other words, the bookends of the season could be mild, but January into early February are more uncertain. Cohen said there has been a growing trend in recent years — likely driven in part by increasing greenhouse gas concentrations — for the polar vortex to remain strong in December, before unleashing shots of cold in January and February.
Cohen also noted Siberian sea ice coverage, which has bottomed out at record minimum levels. He believes that, by heating the adjacent atmosphere, the resulting “ridge” of warm, high pressure over Europe and Scandinavia can disrupt the polar vortex, speeding its disorganization. That would increase the risk of Arctic outbreaks.
“There’s a naturally occurring wave across Eurasia,” said Cohen, describing regimes of high and low pressure that frequent European and Asian weather maps. He said that pattern is being reinforced by sea ice and snow cover patterns.
“The stronger that wave gets, the weaker the polar vortex. A stronger wave gives off more energy, and that … energy gets absorbed in the polar stratosphere and leads to a warming” of the polar vortex, said Cohen. That hastens its demise.
Overall, the experts concurred that warmth was likely over the Lover 48 to kick things off in December, but how the vortex evolves into January remains a mystery. That said, there are prospects for a milder spring.