Why an unusual phenomenon will cause a frigid December in the US

December may be extra cold in parts of the United States this year due to a weather phenomenon happening miles above the North Pole.

This is called sudden stratospheric warming and occurs on average once every two winters, but very rarely in November. Despite the name, its occurrence does not mean that there will be a sudden warming where people live. In fact, it’s almost the opposite.

Rapid warming and weakening of winds in the stratosphere can disrupt the polar vortex, a ring of frigid, stormy air typically high above the North Pole.

Impacts can flow downward through the atmosphere, sometimes causing very cold air to flow south into more populated areas as the polar vortex moves or elongates.

This process may take weeks to unfold, but long-term forecasts favor below-average temperatures in parts of 35 states from late November through December, especially after Thanksgiving.

How and why it happens

Sudden warming of the stratosphere is a natural phenomenon that occurs on average once every two winters.

But major events in November are very unusual; only one has happened since the 1950s – and the event predicted in November could be the earliest ever. They are more common in January and February and not in November or December. That’s because the polar vortex is still in its early stages during late fall and early winter, making it less sensitive to warming events.

For an event to qualify as a major sudden warming of the stratosphere, several meteorological thresholds must be met, including a rapid increase in stratospheric temperatures and a sharp slowdown in winds. A small warming only requires a rapid increase in temperature. These factors can ultimately disrupt the polar vortex, with flow-on effects to weather patterns lower in the atmosphere.

“The reason we pay attention to sudden stratospheric warming is not only because the weather effects can occasionally be severe, but also because changes in stratospheric winds can lead to persistent changes in the weather over many weeks, giving us more time to prepare,” said Amy Butler, a research scientist at the National Oceanic and Atmospheric Administration’s Chemical Sciences Laboratory.

In September, a rare sudden warming of the stratosphere occurred over Antarctica, which tilted the polar vortex and changed weather patterns in the Southern Hemisphere. These events are less common in the Southern Hemisphere, where there is less land and more ocean, leading to more uniform temperatures and usually a strong polar vortex.

When frigid air can arrive

Stratospheric warming is expected to peak in late November, when temperatures in that layer of the atmosphere rise 40 to 50 degrees above average and winds slow down.

Likely related to this phenomenon, strong high pressure is forecast to build over the Arctic Circle, gradually moving the typically frigid air in that area southward into Canada and the US.

Below-average temperatures are forecast to prevail across the country in late November, around or just after Thanksgiving, with several more waves of cold air in December.

In the weeks following a sudden stratospheric warming, below-average temperatures could hit parts of the U.S., Europe and Asia, with above-average temperatures in Greenland and parts of the Arctic, Butler added.

However, Butler also said these details may vary from event to event. It currently appears that the northern US and southern Canada will bear the brunt of this particular episode.

Simon Lee, lecturer in atmospheric sciences at the University of St Andrews, said that because it is still early in the season, the coldest air masses have not yet fully developed. That can limit how intensely cold it can get.

He also noted a lack of consensus on the link between the phenomenon and climate change.

“Despite their increased media attention, sudden stratospheric warming is not becoming more common and climate models disagree on whether their frequency will increase or decrease due to increased CO2 forcing,” Lee said.

Despite the possibility of an unusually cold end to the year, 2025 is expected to be the second or third warmest year on record for the planet.

Increasing interest in the polar vortex

Many people in the US may remember the polar vortex episode of early 2014. For the Midwest, it turned out to be the coldest winter in more than 30 years. In DC, January 2014 was in many ways the coldest in decades.

It also turned out to be a year in which interest in the polar vortex increased enormously.

Although the term polar vortex seems relatively new, the phenomenon itself is not. Vortex disturbances were first described by German meteorologist Richard Scherhag in 1952 – more than 70 years ago.

Despite the term’s increasing popularity and its association with unusually cold weather, Lee emphasized that the polar vortex exists in the stratosphere and that scientists are cautious about using the term to describe individual extremely cold weather events.