On average, the northern half of the United States is colder than the southern half. The boundary separating the cool polar air and the warm tropical air is called the polar front and is a key feature of our atmosphere. Most of the time the polar front is undisturbed, which lends to calm weather. When the polar front starts twisting and turning we get cyclogenesis, the creation of the mid latitude cyclones which bring all the interesting weather to our landscape. The polar front theory was developed in 1917 by Vilhelm Bjerknes and a team of scientists. The theories proposed almost 100 years ago can still explain much of our weather patterns.
The polar front can be perturbed and these perturbations lead to the development of the mid latitude cyclone (aka low pressure system). Mid latitude cyclones bring with them many of our more extreme weather events like thunderstorms, hail, blizzards, floods, damaging winds, tornados, and as we experienced last winter, bitter bitter cold. Let’s take a look at a surface pressure map from 3PM on January 6th, 2014 and locate our midlatitude cyclone. Contoured below from blue to red is the surface pressure, along with the surface wind vectors. Note the counter-clockwise flow around the cyclone (blue low pressure system just north east of the US).
The counter-clockwise circulation around the cyclone means cold polar air moves southward after the passing of a cyclone. We can see the cold air advancing southward into the states in this plot of surface temperature and wind. The counter-clockwise flow pattern around the cyclone is key as it blows colder air from the polar regions southward into the USA. The freezing line at 32°F extends downwards very far south into the country, and if you recall there were portions of northern Texas getting snowed on this past winter! The blue colors are sub-zero temperatures (more than 32 degrees below freezing!)
So we know that the polar front is the boundary between the cold air and warm tropical air, and that disturbances in this front cause mid latitude cyclones, which leads to storms, droughts, cold fronts, etc. But what actually disturbs the polar front and causes these cyclones to form? Meteorologists have found that cyclogenesis is closely linked to the jet stream. The jet stream is a ‘river’ of air flowing around the earth over 30,000 ft above the surface. When the jet stream stays relatively flat (meaning west to east), the polar front is undisturbed. When the jet stream begins to curve northward and southward, mid latitude cyclones form at the surface. A skilled weather forecaster will always first look at the jet stream to get an idea of what weather will be happening at the surface. For instance, lets take a look at the jet stream at our time of interest, 3PM January 6th, 2014.
Note the very sinusoidal up and down nature of this particular jet stream pattern. Following the wind vectors and colors, you can see that there is a large ridge over the west coast (jet stream turns north), and a deep trough over the midwest (jet stream turns south). We know that surface low pressure systems form near the trough in the jet stream, and also that surface high pressure systems form near a ridge in the jet stream. This orientation of the jet stream caused two surface pressure systems (high pressure to west, low pressure to the east) that favored the invasion of polar air that we call the polar vortex.