What Caused the Stormy "Snowpocalypse" of 2010?

With El Nino hunkered down across the tropical Pacific last year, the people at the National Weather Service in charge of seasonal forecasts were looking at conditions last October that gave them extra confidence in their seasonal outlook for the coming winter in the United States. So how did they fail to foresee the historically snowy, bitterly cold weather that so famously clobbered the mid-Atlantic?

(Take a deep breath now: it is not a scandal or a conspiracy or a commentary on the sorry state of science and has nothing to do with our changing climate one way or the other. It is about how Earth works.)

A new study confirms that the forecasters were blindsided by the appearance of dramatically unusual meteorological conditions that slipped into the seam between what we think of as climate and what we call weather. An atmospheric feature known as the North Atlantic Oscillation -- air pressure conditions that shape the jet stream -- hit record lows that created an unusually loopy storm track that carried cold Arctic storms into the U.S. eastern seaboard and across to Western Europe.


“A negative North Atlantic Oscillation this particular winter made the air colder over the eastern U.S., causing more precipitation to fall as snow," said Richard Seager, a climate scientist at Lamont-Doherty Earth Observatory at Columbia University, in a release describing the new study in the journal Geophysical Research Letters. "El Nino brought even more precipitation—which also fell as snow.”

Click on the image above and watch a video of satellite imagery that captures what Seager is talking about -- the especially heavy snowfalls caused by interplay between cold northern storms crossing the eastern U.S. in February at the same time that heavy El Nino flows of tropical Pacific moisture were coming up from the southwest.

Ironically, the forecasters were right about the effects of El Nino on the winter -- as far as they could see.  Across the western and southern United States, El Nino conditions played out as expected, causing drier and warmer than normal weather in the Pacific Northwest -- Vancouver imported snow for the Winter Olympics -- and wet, stormy weather caused mudslides in Southern California.

But they couldn't see the record-low North Atlantic Oscillation coming as early as last October -- and apparently they never will. El Nino is a climate feature -- a set of ocean conditions that hang around for months at a time. The North Atlantic Oscillation is a child of the atmosphere, and Seager and colleagues observe that it "is not predictable beyond the week or two timescale of weather forecasting."

"Until the NAO can be predicted (which may not be possible), such snow anomalies as closed down Washington D.C. for a week will remain a seasonal surprise," the researchers conclude.

IMAGE/VIDEO: National Oceanic and Atmospheric Administration