Polar Vortex Not Only Culprit for Frigid U.S. December

The arrival of colder temperatures this week across much of America is just the beginning of what will likely be an unusually frigid December.

With help from the Arctic, the polar vortex, and even Siberia, December is set to deliver round after round of bitter cold to the majority of the United States - save the Southwest - and this could lend support to commodity markets.

In recent years, colder Decembers have led NYMEX natural gas futures to sustain higher throughout the winter than the years that end up warmer, as natural gas consumption is dialed up as temperatures drop.

Agriculture is usually not as impacted by cold weather but winter wheat - especially when without snow cover - is vulnerable when the temperatures start nearing minus 10 Fahrenheit (minus 23 Celsius). Outdoor cattle and other livestock can usually withstand subzero conditions, but they will not put on weight as quickly as more energy than usual is dedicated to maintaining body temperatures.

The plunging temperatures come after the warmest-ever October through November period - by a long shot - for the lower 48 U.S. states.

December has not been particularly cold in the United States in recent years, as only five of the last 20 recorded below-normal temperatures. But December 2016 looks to be the first colder-than-normal cap to the year since December 2013.

And although December does not always set the tone for the remainder of winter, the odds are in favor that the New Year will usher in below-normal temperatures for January and February as well.

The Northeastern Pacific high pressure dome along with the weak La Niña pattern in the tropical Pacific Ocean, among other factors, make a good case for the continuance of frequent Arctic air intrusions as the winter trudges on, though they do not provide any guarantees.

Siberia Makes a Visit

While a polar vortex is often blamed whenever a cold outbreak hits North America, it is not the only factor this time.

The polar vortex, a spinning mass of cold air over the pole, delivers icy Arctic air into the mid-latitudes during the winter whenever the vortex is perturbed and subsequently weakens.

Although the polar vortex is forecast to regain strength over the next two weeks, the remnants of a sharp vortex weakening late last month will come knocking at millions of Americans' doors from the Plains to the Northeast in the middle of next week.

But current models suggest that a potential follow-up blast arriving the week before Christmas may be even worse than next week's chill for the Northern Plains and Upper Midwest, which could have high temperatures failing to break zero degrees Fahrenheit for a couple days.

Much of this cold air is coming all the way from one of the coldest places on earth - Siberia.

Siberia is not too far away to assist the Arctic in dropping the mercury stateside. Over the next couple of weeks, frigid Siberian air will be directed clear across the pole into North America, often referred to as cross-polar flow.

Cross-polar flow occurs when air masses deep in the Asian continent are able to travel thousands of miles uninterrupted through the Arctic Ocean and into Canada. The persistent dome of high pressure in the Northeast Pacific will facilitate this cold-air transport and amplify the air flow such that the center of the North American continent becomes the bulls-eye.

Other winter-dictating phenomena in the upper latitudes will also be supportive for the colder North American pattern toward the end of the month. Both the Arctic Oscillation and North Atlantic Oscillation are expected to trend more negative in the week leading up to Christmas, which could enhance the biting cold.

So the many Americans dreaming of a "White Christmas" have a much better chance than normal of getting their wish.