Polar vortex El Niño winter weather turned unusual on Tuesday when the circulation began reversing earlier than normal, breaking a barrier that usually keeps the coldest Arctic air near the North Pole. The disruption is expected to last through Thursday, and weather patterns may take weeks to respond.
The polar vortex is a band of upper-level winds wrapping around the Arctic Circle like a dam. When it weakens or reverses, cold air can spill southward, but the exact way it breaks apart helps decide whether the result is a brief cold snap or a longer stretch of winter weather.
Tuesday and Thursday reversal
Based on the latest data, the circulation’s reversal began on Tuesday and computer model guidance expects it to last through Thursday. That timing matters because disruptions of this extent typically do not show up until the latter half of winter, not this early in the season.
The event also comes during a cool season already dominated by the strongest El Niño on record. Historically warm sea surface temperatures in the eastern Pacific Ocean have already significantly affected conditions around the world, and much of North America can expect warmer-than-normal conditions during a winter shaped by El Niño.
Early 2015 comparison
Extreme polar vortex disruptions have produced sharply different outcomes before. A highly disrupted polar vortex in early 2015 sent a weeks-long burst of cold air sweeping over the eastern US, Chicago recorded its coldest February in nearly 140 years, and In Lexington, Kentucky, temperatures bottomed out at –18 degrees Fahrenheit.
But the current pattern does not translate neatly into a forecast for every place. Weather patterns take a few weeks to respond to a polar vortex disruption, if at all, and reliable forecasts only extend about 10 days into the future.
Much of North America
That leaves much of North America facing a forecast shaped by two overlapping forces: an early polar vortex breakdown and the strongest El Niño on record. Unexpected things can happen when unprecedented phenomena overlap, and where Arctic cold air spills first across the northern hemisphere is not known yet.
For now, the practical read is narrow: the disruption has started, it is expected to run through Thursday, and the weather response may not show up right away.
