A Sudden Stratospheric Warming event.
"At altitudes of less than 10 kilometers above Earth’s surface, planetary-scale waves form and propagate upward into the stratosphere, where they dissipate. This leads to a weakening of the polar vortex, a confined region of strong eastward winds that form during wintertime at high latitudes. As the polar vortex weakens, polar stratospheric temperatures increase"
"Hemisphere-scale weather patterns in the wintertime Northern Hemisphere tropo