The winter season over North America during 2013-2014 was dominated by a persistent ridge-trough that brought warm and dry conditions to the southwestern U.S., and markedly cold temperatures to central and eastern North America. In addition, several cold air outbreaks occurred during the winter season, the strongest of which was around 7 January 2014 and led to minimum daily temperature records being set at many weather stations including Atlanta, Austin, Chicago and New York. This study uses a novel decomposition of wave variability into standing and travelling components to diagnose the anomalous circulation of the 2013-2014 winter season. An index representing the ridge-trough dipole is defined using mid-tropospheric heights and shown to be well correlated with surface temperatures over central and eastern North America. The contributions to this dipole index from standing waves, westward travelling waves, and eastward travelling waves are calculated. The analysis demonstrates that the cold air outbreak of 7 January 2014 was driven by a synoptic wave of extreme large amplitude intensifying a persistent background amplification of the typical ridge-trough structure seen during North American winter. A comparison is made with the more recent 2014/15 winter, which had a similar seasonally-averaged anomalous circulation pattern as the 2013/14 winter.