The cold air that has gripped much of the eastern 2/3’s of the US during the week of January 6, 2014, has the media using the term “the polar vortex.” I have seen the term: “a dive into the polar vortex.” This is certainly is a term that gets everyone’s attention.
What is a vortex? It is a region within a fluid where there is a spinning motion.
Let us look at some more details. What seems to be in play is the Arctic Oscillation (AO) which swings from positive to negative phase. We will focus on negative phase since that is what is happened the week of January 6, 2014.
What happens in the negative phase of the AO? The pressure over the Arctic is weaker than normal and one result is the upper flow in the mid-latitudes at about 18,000 feet tends to be weaker. So instead for the normal zonal flow (from west to east), we get a more meridional flow (from north to south). This is due to a higher pressure to the north lower to the south so wind blows from the north to the south. Thus, extremely cold conditions.
How about the West Coast? During much of this period, the West Coast has been locked into a ridge of high pressure that has been stationary for the late fall and early winter. Thus, much below snowpack. I know many skiers hope this changes soon. Some suggest that these blasts of cold air help keep the ridge of high pressure in place over the Northwest. Time will tell.