Stratospheric Warming is in simple terms just that. When the air temperature warms suddenly in the stratosphere in The Artic of North America. This is where the polar vortex is located. This usually happens quickly within usually a weeks period of time where temperatures quickly warm in the stratosphere. This causes one of the most drastic of weather changes, as it at times can make the NAO (North Atlantic Oscillation) turn negative. And misplaces the polar vortex which usually stays confined up into Northern Canada and in the Artic circle.
What causes this sudden warming? When The winds in the winter hemisphere slow or completely reverse direction which cause the warming to take place. While this happens every winter, some winters this warming is more significant than others. The final warming event usually occurs in March trough April and thereafter the Polar vortex usually dissipates.