The snowstorm in the Northeastern U.S. that dropped several inches of snow was the partly the result of an unusual combination of a cool Pacific Ocean and smaller pressure differences between the North and South Atlantic.
Jeff Masters, Director of Meteorology and one of the founders of Weather Underground, Inc. a weather information site, noted that ordinarily a warm Pacific generates high pressure, and that pushes the jet stream northeast. The high pressure zone over the southern Atlantic and the low pressure zone over Iceland drive the jet stream nearer to the North Pole, blocking the colder air from the northeastern U.S.
In most years the winter storms travel a generally U-shaped path that takes them from the Pacific Northwest to the Great Plains and then out over eastern Canada. It takes until late January or early February for the air in the Arctic to get cold enough that the jet stream inches south far enough to bring nor'easters -- and lots of snow -- to New England and the Mid-Atlantic states.
This year was different, Masters said, because the La Niña in the Pacific made the region cooler, lowering the pressure and allowing the jet stream to move south. Over the Atlantic, the North Atlantic Oscillation, which describes the difference in the pressure between the North and South Atlantic, was positive, meaning that it there wasn't a wall of high-pressure air stopping the jet stream and trapping storms farther north.
This was very unusual, he said. We got a lot of things you usually don't get in La Niña years.
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As the atmosphere was changing configurations, the storm had a chance to reach areas it ordinarily would not.
Another effect of the storm was thundersnow. If you heard thunder and saw lightning you weren't imagining things, Masters said. To get thunder and lightning all you need is a big temperature difference between the lower air and the upper air. Usually that happens in summer. But it can happen in winter as well because it is the difference in temperature, not the temperature itself, which matters.
The good news, he said, is that the new configuration looks like it will be more normal, putting off any more big snowstorms to their usual late January-early February time frame.
One thing he stressed is that this particular weather event - or any other - doesn't have much to do with theories of global warming. When one speaks of average global temperatures, it doesn't necessarily mean that winters will be so much warmer that people will feel it. We've raised the temperature by about 2 degrees Fahrenheit this century, Masters said. It's still cold in winter. Winter isn't going away.
If anything, there's a case to be made that winters would be wetter and possibly colder in some places, with more snow dropped in a shorter season. That's because in a warming world, the difference in temperature between the Arctic and the equator is smaller. That means the jet stream is weaker and the cold Arctic air from the north can more easily flow south.
What will likely happen is that the spring will come earlier and the number of days where it is too cold to snow will go down. That could mean more snow, rather than less, at least in some areas.
masters added that snowstorms like this one would be less common later in the year. If this thing had happened in March and it was a few degrees warmer you'd have a rainstorm.