To say that January has been a tumultuous month weather-wise would be an understatement. The month has featured a string of severe winter storms, wild temperature swings and even a few bouts of tornadoes. And as we go into February, an even trickier complication enters the equation: the ever-capricious polar vortex.
Over the past week, Arctic cold has swallowed much of the nation’s heartland. Thirty states were included in alerts for dangerously low temperatures and wind chills, with readings dropping to minus-16 in Kansas City, 1 degree in Oklahoma City, minus-10 in Chicago and minus-27 in Bismarck, N.D. Snow cover reached its greatest mid-January extent over the contiguous United States in the last 20 years.
After one more frigid weekend, temperatures are about to spike 30 to 40 degrees, rising above freezing and inducing an abrupt thaw over the Ozarks, southern Plains and Mid-South. The northern Plains will see highs spike 15 to 20 degrees above normal by Tuesday.
But after that warm-up, then what? The jury is still out on what might happen. Much of it will boil down to what the polar vortex does, and how its shape and intensity evolve to affect the weather in North America. The ongoing El Niño may play a role as well.
Why the polar vortex matters
There are two polar vortexes in the Northern Hemisphere: one in the lower atmosphere, or the troposphere (sometimes called the “circumpolar vortex”), and one higher up in the stratosphere. The stratospheric one is stronger and is present each winter as cold, dense air sinks and spirals near the North Pole. The stronger the vortex, the faster it spins and the more bottled-up the cold air is over the high latitudes.
But if something disrupts the polar vortex, it can be knocked off-kilter. Imagine a spinning top; if you jiggle the table, it might wobble out of control. If the polar vortex weakens, it can no longer contain the cold air — and pockets of frigid Arctic chill crash south over North America.
What the polar vortex has been doing
According to Climate.gov’s Polar Vortex blog, there was a subtle disruption to the polar vortex a couple of weeks ago. Temperatures inside the vortex warmed a little bit, and the whirlpool of air slowed down its spin. It didn’t reverse direction initially, which is the criterion for a major disruption, but it did slow markedly. Such minor disruptions are “not unexpected,” the Climate.gov blog post said.
Is that enough to affect our weather in the United States? Ordinarily no. Minor stratospheric disruptions happen 19 miles up, and by the time the signal propagates to the surface, it’s too weak to actually do anything.
But this last episode was wonky. Climate.gov stated that it occurred at the lower levels of the stratosphere, allowing it to warm up and destabilize. “It appears as though the minor [vortex] warming during the first week of January and the subsequent destruction of the polar vortex in the lower stratosphere were enough to at least help set the stage for the cold air outbreak over North America this past weekend,” Climate.gov wrote.
But what comes next?
The polar vortex’s next move
Meteorologists believe that polar vortex disruption a couple of weeks ago in the lower levels of the stratosphere could help energize a stronger disturbance higher up. The jostled polar vortex would briefly collapse, filling in with warm air and even briefly reversing its spin. That theoretically could let the jet stream, or river of winds high up, shift south, spilling the cold over the Lower 48.
But don’t count on any big wintry weather anytime soon. Here’s why:
- The polar vortex collapse would take a little while; surface impacts, where we live, aren’t immediate following such a disruption. Any influence on the weather in the contiguous United States would take time.
- The polar vortex disruption will probably be brief. Brad Pugh, a forecaster at NOAA’s Climate Prediction Center, wrote in an email that “the [disruption] is expected to be short lived (a couple of days) and winds [in the vortex] will strengthen again toward the end of January.” That could eventually favor more mild air in the mid-latitudes.
So basically, the polar vortex has the stage set for another wave of cold if other factors line up, but it’s not guaranteed, and mild weather could end up prevailing. And there are other atmospheric processes that will play a role in how the weather unfolds.
Pugh said he’d keep a close eye on El Niño.
Putting it all together
We know for sure that next week will be mild over the Lower 48 states. After that, the forecast is a lot more cloudy.
Some forecast models project that the Arctic Oscillation, which approximates the strength of the polar vortex, will intensify into next week and weaken the week after, shifting toward its negative phase. A negative Arctic Oscillation — since it reflects a weak polar vortex — tilts the odds for more cold air to spill into the mid-latitudes.
Meanwhile, as we head into February, a weakening El Niño means there are elevated chances for above-average snowfall in the eastern United States. There’s a link between a weakening El Niño and an uptick in East Coast snows late in the wintertime.
“As far as El Niño, I do think that less than strong is better for snow and cold in East,” Judah Cohen, an atmospheric scientist and specialist in long-range weather forecasts at Atmospheric and Environmental Research, wrote in an email.
Cohen is also watching the polar vortex. “If it stays strong and circular, it is mild for February,” he wrote. But if it stretches out, “we get episodes of more severe winter weather.”
For now, February looks to start off warm. And while there are no immediate signs of sudden flare-ups of winter weather, it’s important to remember that, between El Niño and the effects of this polar vortex disruption, the bases are loaded. It wouldn’t take much for winter to come back with a vengeance.
Jason Samenow contributed to this report.