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In the last 20 years, the latest cold spell occurred March 10, in 2003.
Madison is in the middle of a frigid stretch that may make a run at some rather obscure signatures of winter cold from the past.
The polar vortex is a band of strong winds, high up in the atmosphere that spins counter-clockwise around the North Pole.
Whether or not we have just had our last below zero night of the season, we really do not have that much farther to go.