By Nicolas Rivero and Daniel Wolfe, Quartz
It’s been an odd winter in the US. Except for a brief polar-vortex-induced cold snap that plunged swaths of the northeast and midwest into frigid temperatures, much of the country has experienced a relatively mild winter.
Yet in the past month, four snowstorms hit the Pacific northwest in nine days and Seattle ran up the highest February snowfall totals in decades. In the usually arid southwest, Flagstaff smashed its single-day snowfall record and Las Vegas recorded a measurable amount of snow for the first time in a decade. The snow anomalies reached all the way to Hawaii, where Maui saw snow at the lowest elevation ever recorded.
Many news accounts have suggested climate change might be at work, supercharging extreme winter weather. The full explanation, meteorologists warn, is a little messier.
“The simple way to put it, and it fits disgustingly well in this case, is shit happens,” said Washington state climatologist Nick Bond.
The main culprit is a high-pressure system—which pushes air away from its center and generally creates lower temperatures and clear, cloudless skies—that formed off the coast of Alaska at the beginning of February. The system created a bend in the jet stream, the band of air that usually spins an uninterrupted loop around the Arctic. As a result, cold air from the Arctic got sucked south along the West Coast, while warm air from the midlatitudes went north over Alaska.
There is some evidence that climate change could make the jet stream more likely to meander. Normally, the jet stream follows a straight path between cold air in the Arctic and warmer air in the mid-latitudes. But as the polar regions heat up, the temperature difference between the Arctic and mid-latitudes is getting smaller, and the jet stream may be getting wavier.
In any case, this winter, air swirled around the high-pressure system off the coast of Alaska, picking up moisture over the Pacific that fell as snow when it passed over the northwest. This weather pattern stuck around throughout February, growing strongest in the middle of the month when it whipped snowstorm after snowstorm into Washington.
“Sometimes these systems can set up and be there for five days, and other times they’re there for 30 days,” said Bond. “There is just a very important random quality to that.”