THURSDAY MARCH 29 - NORTH ATLANTIC OSCILLATION: PART 2


Last night I introduced the North Atlantic Oscillation, or NAO. This cycle has two extremes, called the positive and negative phases. Research has shown that when the NAO is in its negative phase, there's a greater chance of cold and snow in the East. Here's why.

In the negative phase, the high altitude winds that move weather systems across the Atlantic Ocean tend to be relatively weak, and the jet stream pattern tends to have lots of dips and bends in it. The weaker winds mean that cold air in the Eastern U.S. doesn't move out to sea very quickly, keeping the chill on land. The wavy jet-stream pattern tends to promote stronger storms. So colder air plus a tendency for strong storms means more potential for snow.

Here's a graph showing the NAO the last couple months. Times when our area got at least one inch of snow are marked with a snowflake (*). It's far from a perfect correlation, but in general the snowy times tend to occur when there’s a negative value of the NAO. So if we can predict the NAO, it's at least a heads-up to forecasters for times to be especially watchful for snow. I promise, you'll be hearing a lot more about the NAO in winters to come.

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