Why has it been so cold in the western USA lately?

The western United States experienced an extremely mild, dry winter during 2011-12.  This year (2012-13), however, things are different - it's cold, cold, cold! and wetter than last year, too.

For example, where I live in SE Idaho, we have been experiencing low temperatures in the -10 to -20oF range over the last week or so.

Why is it so much colder than last year?

Last year's mild winter can be explained in part by ongoing global warming, but prevailing weather conditions over the Arctic also matter.

There is something called the Arctic Oscillation (AO).  The AO has two phases, a positive and a negative phase.  During the Positive Phase of AO, low pressure sits over the Arctic and a high pressure system dominates around 45oN.  During the Negative phase of AO, the opposite occurs; a high pressure system dominates the Arctic and low pressure exists around 45oN.

You can learn more about it at this site provided by the National Snow and Ice Data Center (http://nsidc.org/arcticmet/patterns/arctic_oscillation.html)

The bottom line is that the AO can switch between its positive and negative phases over a matter of weeks to decades.  The dominating phase of AO can have a significant impacts on the weather over the northern hemisphere.

 (Images courtesy of NSIDC.org)

The image above left shows the effects of the "Positive Phase of Arctic Oscillation".  During the positive phase (low pressure system over the Arctic) masses of cold Arctic air stay farther north, and the western USA stays warmer and drier than usual.  At the same time, coastal Eastern Canada gets colder air than usual.  During the Positive phase the North Atlantic storm track can also move farther north than usual, bringing cold, wet winter weather to northern Europe.  According to NSIDC.org, we have been experiencing mainly the positive phase of AO since the 1970s.   

The image above right shows the effects of the "Negative Phase of Arctic Oscillation."  During the negative phase (high pressure over the Arctic), low pressure is much more common around 45oN, and this phase recently developed.  During the negative phase colder, wetter air masses than usual are pulled farther south by the low pressure system over western North America.  Along with this we normally see the Atlantic storm track pushed farther south, bringing precipitation to the Mediterranean instead of northern Europe.  

There are, of course, many other oscillating weather patterns that contribute to conditions we experience, but the AO is one recently switched phases.

Global Warming also plays an important role in generating weather.  Even though the Arctic is cold, much more heat than normal is stored there, and it has to go someplace.  One thing this does is increase the amount of atmospheric activity and can contribute energy to the polar jet stream, pushing it farther south than usual.  The map below shows the jet stream track for 14 January 2013:

The map above shows the jet stream pushing as far south as northern Mexico.  My son happens to be there, and he reported recently that they are experiencing freezing temperatures and even snow!  

Knowing something about the Arctic Oscillation effects of global warming help us understand weather we experience.

I hope this was helpful.