Tourist on Riverside on a sunny but cold day in London on Nov. 30, 2016.
Image: AP

The fall of 2016 has been astoundingly warm across much of North America, but indications are that this is about to change in a big way.

So, consider yourself warned.

November will go down in the record books as one of the weirdest such months on record, with North America’s climate stuck on “roast,” at least compared to average temperatures for this time of year, the Arctic setting records for unusual warmth and low sea ice coverage, and Siberia locked in snow and cold that is even frigid for, well, Siberia.

It now appears that some of that cold, though not the heart of it, is poised to slosh eastward and then south, into Alaska, Canada and the U.S.

In other words, get ready for colder, snowier weather in early to mid-December.

Forecast temperature anomalies in early December.

Image: weatherbell analytics

As with most long-range forecasts, the details are subject to change, but much of December is looking far different for the U.S., particularly in the West, compared to November.

This is definitely a huge cold air outbreak for the western U.S.,” said Jason Furtado, an atmospheric scientist with the University of Oklahoma. “Were talking temperatures 12 to 15 degrees Celsius below average for a few days in the West.

This is going to be a real shock to the system.

To give you an idea of how out of whack November was, and therefore how significant the temperature reversal will be, consider that through Nov. 29, the U.S. saw 4,403 record high temperatures set or tied, compared to a measly 80 record lows.

This continues a long-term trend toward more warm temperature records compared to cold records, which is tied to human-caused global warming, urban heat island effects and other factors.

Temperature anomalies during the month of November.

Image: weatherbell analytics

December is looking very different, however. At the very least, the West is in store for colder than average temperatures during the first two weeks of the month, and this should allow ski resorts from Colorado to California to build up a solid base of snow cover to ensure a successful start to the season.

According to Furtado, the warmth during the fall was partly the result of a strong, “zonal” jet stream across the North Pacific Ocean, meaning that winds were blowing almost straight west to east.

To get Arctic air outbreaks in North America, the jet stream needs to meander, coiling back and forth like a snake to form what meteorologists refer to as ridges and troughs.

This is what is about to happen, Furtado said, with a ridge forming off the southwest coast of Alaska. This will force cold air south into the U.S. by carving out a dip, or trough, in the jet stream across that region.

The cold will eventually spill eastward, if recent computer model projections are correct. All of the lower 48 states could see colder-than-average temperatures for at least a few days during the middle of the month. The coldest temperatures could be at least 20 degrees Celsius, or 36 degrees Fahrenheit, below average.

Because it’s been so unusually warm in recent months, such an episode of weather whiplash could feel especially painful.

Its gonna be a whiplash for sure, Furtado told Mashable in an interview.

The weather pattern during much of December is also likely to be stormier than it has been in recent months, raising the possibility that the meager snow cover now present across the U.S. and Canada will get a significant boost.

What about the long-term?

On a broader scale, forecasters are keeping their eye on what happens with two weather features that will play leading roles in determining if western Europe and the big cities of the Northeast U.S. have a cold and snowy winter overall: the polar vortex and a pool of dense, ultra-cold air anchored over Siberia.

The main polar vortex is a circulation of air enveloping a near-permanent area of low pressure that exists in the upper atmosphere, above typical cruising altitudes for commercial jetliners, over the Arctic.

When these winds weaken, as has been happening recently, pieces of the vortex can break off, and drunkenly wander south into the U.S., Europe and parts of Asia. Lately, the weak polar vortex has featured a main center over Eurasia, where it has been extremely cold and snowy.

The vortex is forecast to intensify and retreat back toward the North Pole during December, which would tend to lower the odds of any historic cold air outbreaks in the U.S. and western Europe.

However, that pool of frigid air over Siberia needs to be watched, because if given an opening, it could be transported toward North America or parts of Europe, turning the winter of 2016-17 into another memorably cold and snowy one for tens of millions of people.

BONUS: Biggest die-off of corals ever recorded on Great Barrier Reef

Read more: