Polar vortex collapses earlier than ever, clouding winter forecasts

What has happened

The polar vortex, a band of high-level winds that circles the Arctic, has broken down earlier in the season than at any point in the historical record. According to WIRED, the winds began reversing on Tuesday and computer models expect the disruption to last through Thursday.

Such breakdowns usually happen in the second half of winter, not in November. The event is a sign of an unusual season ahead, although experts cannot yet say how it will affect day-to-day weather.

How the polar vortex works

The vortex acts like a dam around the Arctic Circle. When it is strong and stable, it keeps the coldest air locked near the North Pole. When it weakens or reverses, a chain reaction can let bitterly cold air spill southwards.

How the vortex breaks down matters. If the winds dip southwards in troughs, regions see short bursts of cold. These lobes are common in a normal winter.

If the circulation instead splinters into fragments, the cold can park over the same places for days or weeks. WIRED compares these pieces to leaves caught on a riverbank, holding freezing air in place.

Why past breakdowns mattered

A heavily disrupted vortex in early 2015 shows the potential impact. It sent weeks of cold air across the eastern United States, and Chicago recorded its coldest February in nearly 140 years.

During that event, Lexington in Kentucky reached minus 18 degrees Fahrenheit, and subfreezing air pushed as far south as the Gulf of Mexico.

Why forecasts are uncertain

Weather patterns take weeks to respond to a vortex disruption, if they respond at all. Reliable forecasts extend only about 10 days ahead, so nobody can yet say where Arctic cold may end up in the northern hemisphere.

Experts are still debating what caused this early collapse. The event also lands on top of an already exceptional year: WIRED reports this is the strongest El Niño on record, a pattern of unusually warm sea surface temperatures in the eastern Pacific that reshapes weather worldwide.

What it means for the winter

A strong El Niño typically brings warmer-than-normal conditions to much of North America, and the Pacific's influence will grow as winter goes on. But the vortex disruption makes those outlooks less certain, because overlapping unprecedented events can produce unexpected results.

For readers in the UK and US, the practical message is to expect a less predictable winter than usual and to keep an eye on forecasts as the season develops.