MEMPHIS, TN (WMC) - The upper level winds, known as the jet stream, can bring in cool air from the north or push warm air from the south into our area. The jet stream sits 6-9 miles above the surface of the earth, but it impacts our weather all the way down at the surface.
There are two main jet streams that fluctuate based on the time of year. In the summer, the subtropical jet typically sits over the southern United States, while the polar jet is located in the northern United States. Since there is not much typical variation at the surface during the summer, the jet streams are much weaker and less active.
During the winter, there are large variations in temperatures between the much colder north and warmer south. The polar jet usually moves farther to the south in the winter, which is why we get cold blasts from December to March.