There can be multiple polar vortices

There's a lot of chatter about the polar vortex sliding off the north pole into NYC. The reply from meteorologists is, "which polar vortex do you mean?"

The northern hemisphere (NH or Boreal) polar vortex is a bit more irregular than the southern hemisphere's (SH or Austral) due to greater anisotropy of land masses near the north pole.  See NOAA's Arctic Climate and the Polar Vortex page.

500 mb NH wind speed (screenshot http://earth.nullschool.net)

Cameron Beccario has written a beautiful visualization of global wind model data from the models run by NOAA/NCEP.  I took screen grabs of the animation for 500 mb (half height of the atmosphere) and 250 mb (near the vertical peak of the jet stream).  Notice one of the polar vortices (N or Russia) rotates clockwise instead of counterclockwise!

250 mb NH wind speed (screenshot http://earth.nullschool.net)

A fast-moving river of air acts like a barrier or separatrix between regions that can have very different characteristics.  But, you can see that this fence is kind of irregular and spotty.

Occasionally, the vortex becomes VERY wavy and dips down into mid-latitudes, bringing cold air along with it.  These episodes are often associated with a sudden stratospheric warming (SSW).  SSWs and mid-latitude cold blasts happen every few years (more if you count both poles).

I feel your pain and will think of my midwestern and east coast friends while I sip coffee in my shorts at the beach today.

People better qualified than me have explained the connection between SSW, arctic cold air masses, and how a single event like this is not an indicator of global warming.

24hourprof
Cliff Mass is particularly vehement
Jennifer Francis suggests that reduction of arctic ice leads to more waviness in the polar vortex
Elizabeth A. Barnes sees no net increase in waviness over 1980-2011

NASA on the SSW in January 2013
NASA background on SSW and Ozone

The Barnes article is a Geophysical Research Letter (GRL), a short communique to disseminate preliminary research.  I think that a 31 year study is a good start, but I withhold judgement until I see more evidence and a longer-term study.