Why Was December So Cold?

Frigid weather has been hard to come by the past few years in the Pacific Northwest. Due to the “Blob” of warm water in the Northeast Pacific for much of 2014 and 2015, a very strong El Nino during the 2015-2016 winter, and additional warming due to climate change, Sea-Tac Airport has only recorded two months of below-average monthly temperatures since January 2014 (February 2014 and November 2015). But this December, we’ve seen our fair share of winter weather, with sub-freezing temperatures, lowland snow and even freezing rain in spots around the Columbia River Gorge. What changed, and why have we been so cold?

The culprit behind our cold December is also every news station’s favorite winter weather buzzword: the “polar vortex.” But the “polar vortex” wasn’t just conjured up by a creative journalist some blisteringly cold winter day; it is an actual phenomenon in scientific literature that only found its way into the mainstream media over the past couple years. The polar vortex is an area of low pressure and very cold air that is generally kept locked up in the arctic by the jet stream – the current of strong winds in the upper troposphere that steer the paths of storms across the mid-latitudes. But sometimes, a strong ridge of high pressure will push the jet stream poleward, disrupting the polar vortex and weakening it. As the vortex weakens, it loses its staying power in the arctic and can actually slide down into the mid-latitudes courtesy of a large trough in the jet stream, bringing true arctic air down to the Lower 48.

Early in December, a ridge of high pressure started to bulge northward into Western Alaska/Far Eastern Russia, disrupting the polar vortex and sending it southward. One cold wave came through early-mid December with highs in the upper 30s and even an inch of snow between December 8-9, and an even colder one came in during mid-December with highs struggling to reach freezing in many areas throughout Western Washington.

But as cold as things were here, they were much colder further east. Casper, Wyoming dropped down to -33 the morning of December 8, and West Yellowstone, Montana dropped to a bone-chilling -43 degrees on December 17. We Seattleites can thank the Rockies and the Cascades for blocking this truly frigid air from entering our region.

Some long-range models hint that January may look like December, with the potential for occasional arctic blasts and bouts of light snow for parts of the Pacific Northwest. Be sure to check out my weather blog at charlie.weathertogether.us for all the latest on the wonderful weather we experience in the Pacific Northwest, including the potential for snow this month!

~Charlie Phillips

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