Cold + Snow in March = Crabby People. Why is it so cold?

If you are like the many people I've heard from in the past week or so, you are very sick of the cold and snow that the area has seen over the past few weeks.  Many people are longing for the warmer and snow-less days of spring that typically begin around this time of year.  However, we have been stuck in a cold and snowy pattern that doesn't seem to let up.  This has led to many crabby people around the region!  And it has left many wondering why.

Negative and positive phases
of the Arctic Oscillation

It all has to do with the Arctic Oscillation (AO).  The AO is an index of sea-level pressure variations north of 20°N latitude and is either given a positive or negative value.  This index is directly related to the degree to which Arctic air can penetrate into the mid-latitudes, where we live here in the Midwest.  When the AO is positive, this symbolizes low pressure in the Arctic.  This allows the mid-latitude jet stream to blow strongly and consistently from west to east in what is known as a zonal or near-zonal flow.  This keeps the cold Arctic air locked into the polar region.  However, when the AO becomes negative, this means that there is high pressure in the polar region, resulting in larger swings of the jet stream (much less zonal) and allows for greater movement of very cold polar air into the mid-latitudes.  These large swings in the jet stream also result in strong storms sliding along the stream which can produce several inches of snow at a time.

Well, right now, the AO is negative.... very negative.  As the chart below shows, the AO is currently at its lowest level of the winter with an index value near -5.5!  This means that the high pressure over the arctic is strong, and is sending plenty of cold air southward into the eastern half of the nation (as you are probably well aware of)!

The Arctic Oscillation Index during the 2012-2013 winter months. The black line is the observed number, while the red lines are the predicted values for the near future.

Luckily, the AO index is predicted to rise dramatically through April 1st, which, if correct, means that the cold air will be retreating back into the polar regions and more spring-like air will move into the region.  Rejoice spring lovers!  Granted, it does not mean a complete reversal of the pattern, but it will mean that the cold conditions we are seeing across the eastern half of the country will ease up.

The April temperature outlook (left) and precipitation outlook (right) released by the Climate Prediction Center on Thursday, March 21st, 2013

And the April outlook released today from the Climate Prediction Center tends to agree.  They are anticipating slightly above average temperatures for the eastern and southern portions of the United States during the month of April, with below normal temperatures along the west coast.  And if you are wondering about precipitation, it appears the Great Lakes is expected to see above normal amounts of precipitation while the coastal areas may see below normal precipitation.

So it appears that the month of April should be near normal temperatures but wet for the Midwest.  So, yes spring lovers.  Rejoice.  Spring is on the way!