January 10th, 2010: Welcome Pattern Change ( Brian Bledsoe )

After such a cold December and a pretty cold first week of January, we are finally going to keep the Arctic Air away.  For a while...  The reason?  A big swing in the weather pattern and the various oscillations that control our weather changes.  If you read our blog regularly, you know that we've been talking about the Arctic Oscillation ( AO ), the Pacific North America Oscillation ( PNA ), and the North Atlantic Oscillation ( NAO ).  The two oscillations that are important for this blog, are the AO and the PNA.  Below is the AO index for the past several weeks and the subsequent forecast for the near future:

The AO went strongly negative early in December and has just now started to come back toward neutral territory.  Remember, when the AO goes negative, it allows bitterly cold Arctic Air to readily spill southward into the US.  Fortunately for Southern Colorado, we do not have a deep snowpack and the core of the coldest air has gone to our east.  Otherwise, this may have ended up one of the coldest Winters on record for us...  Now that the AO isn't so negative, we should have a relatively mild period of weather for the next 7 to 10 days and maybe longer...

The PNA has also started to change phase.  Check it out below:

The PNA has largely been positive for the past several months.  Thus, we haven't really had many storms come from the southwestern part of the country.  When the PNA goes negative, it allows storms to move into the southwestern part of the US, and eventually roll eastward into the Plains.  Many times, our biggest storms occur when the PNA is negative.  Also, when the PNA goes negative, it allows a milder Pacific flow to ensue.  As you can see above, the PNA is forecast to become more negative in the coming days.  This will likely mean our weather pattern will get more active, within the next couple of weeks, with storms originating farther south and potentially having more of an impact on Southern Colorado.  While January and February are typically not wet months for Southern Colorado, March and April are right around the corner and could prove to be quite active if the PNA swings toward a prolonged negative phase. 

Should be interesting to see, but for now, I'm glad the Arctic Air has retreated for a while...

Chief Meteorologist Brian Bledsoe

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