The North Atlantic Oscillation is one of the most important factors in winter. By definition, it is the measure of fluctuations in the difference of pressure at sea level. This is the pressure differences between the Icelandic Low and the Bermuda-Azores High.
The NAO can be either negative or positive. When it is negative, it is more likely for there to be snow storms and nor'easters on the east coast of the US and across Europe. For example, the winter of 2009-2010 was a historic winter, breaking records across the eastern US and Europe. In the eastern US, there were 4 blizzards during that winter season. Normally, this area would only receive one blizzard per decade. In Europe, they had the coldest winter in 30 years. When it is positive, the eastern US receives significantly fewer snowstorms than when negative. This because the +NAO causes warmer temperatures, and therefore more rain.
The NAO also influences storm track. When it is negative, storms on the east coast will more likely go up the coast, possibly causing a snowstorm. But when positive, few storms go up the coast, and when they do, they are usually rain.
There still are snowstorms in the Eastern US during times of a +NAO, but blizzards are rare.
NAO Indices since 1860:
Correlation between NAO and Winter Storms on the East Coast:
Not pictured is the Blizzard of 1899, Blizzard of 1922, and the Blizzard of 1969.
There is a big correlation between the two. Almost all blizzards occurred during a -NAO, the only exception being the Blizzard of 1993.
If you find yourself tracking a winter storm in the long range, remember to look at the NAO. You can find forecasts for the NAO at:
http://www.cpc.ncep.noaa.gov/product...nections.shtml
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