November 2005 was very active for tornadoes across the United States, with three large outbreaks and one additional significant tornado (see the List of tornadoes and tornado outbreaks). For much of December 2005; a cold, stable Arctic air mass prevailed over much of the United States east of the Rockies. However, by the start of the new year the pattern began to reverse itself, and moist, unstable air once again flowed northward from the Gulf of Mexico (a pattern normally reserved for spring) and combined with a series of strong low pressure systems tracking across the country, conditions were present for tornado formation.