The Polar Vortex pays a visit
The polar vortex is an area of cold air with low amospheric pressure in the Arctic. It exists year-round, its winds swirling counterclockwise and inward towards the North Pole. The polar vortex is constrained by the polar jet stream, a narrow, fast-flowing current of air in our upper atmosphere circling the Earth between the Arctic and mid-latitudes.
So what sent it south? Global warming. The Arctic is warming much faster than other areas of the planet, making the temperature difference between the vortex and the jet stream less distinct. This allows the polar vortex to push the jet stream’s border up and down, driving arctic temperatures south in some areas while hotter temperatures can climb further north in other areas.
Source: NOAA, UCAR Center for Science Education