November 24, 2009: Warm November, Cold December?

Through today, Colorado Springs is 5.3 degrees above normal in the temperature department for November.  Pueblo is 4.7 degrees above normal for the month.  This in stark contrast to the month of October, when it was the coldest October on record for Pueblo.  There is a reason for this, and the reason isn't just about El Nino.  Granted, El Nino did provide for a supercharged storm track in October and gave Colorado several chances for precipitation and a few big snow storms to boot.  But something else is there... it is another oscillation... it is the Arctic Oscillation.  Check out the graphic below:

At the top of the graphic, you see that the Arctic Oscillation is measured in an index.  Without getting too technical, when the AO is positive, we are usually pretty mild and the Arctic Air is bottled up to our north.  When the AO is negative, we are usually colder than normal, as the colder air to our north can come south and impact us.  Notice how most of October was dominated by a negative AO.  Notice how the AO went strongly positive early in November and has pretty much stayed positive the whole month.  Thus, we've been pretty mild the whole month of November.  Usually during an El Nino, the AO stays rather positive and we don't see a lot of Arctic Air.  This may change for the first two weeks of December.  The bottom 3 graphics in the above graphic show the forecast for the AO.  Notice that it goes pretty negative as we enter December.  Because of this, I look for some pretty cool air to control the US and possibly including us here in Colorado, for the month of December. 

Below is a graphic that also shows that a colder stratosphere ( layer of the atmosphere above us here in the troposphere ), promotes a positive AO and keeps the cold air bottled up to the north.  Conversely, a warmer or less cold stratosphere, means a negative AO and allows cold air to move into the US.  Pretty cool...

What does this mean for snow?  Tough to say.  However, during the presence of an El Nino quite a bit of mild Pacific air comes into the southern US.  If the AO goes negative, this may allow a stronger battle of the airmasses.  Arctic air to the north and Pacific air to the south.  This battle sets up an active storm track and allows for storms to feed on the differential energy of the battling airmasses.  Personally, I think this will spell a pretty active mid to late December.  Given how nice and mild November has been, it will take a week or two to break the pattern.  Once it breaks, it may stay active for a while.  We'll see...

El Nino and the AO are just two of many different Oscillations that impact us.  I will reference several here in our blog, but if you want to know more about them, go here:

Chief Meteorologist Brian Bledsoe


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