Sudden stratospheric warming
Sudden stratospheric warming (SSW) is an atmospheric phenomenon that occurs when polar stratospheric temperatures suddenly rise by several degrees (sometimes as much as 50 °C (90 °F)) over the course of a few days. SSW's occur high in the stratosphere, are often associated with Rossby waves and Polar Vortex breakdown and come in varying magnitudes.
Source: Wikipedia — Sudden stratospheric warming (CC BY-SA 4.0)