In a, series of idealized numerical experiments, Butchart et al (1982) have recently established that the configuration of the mean zonal wind occurring immediately before the wavenumber-2 major stratospheric warming of February 1979 was crucial in subsequently focusing upward propagating planetary wave-activity into the high latitude stratosphere. In this sense, it was concluded that the stratospheric circumpolar flow should evolve to some preconditioned state before a wavenumber-2 major warming could occur. In the present paper, the mechanisms responsible for the transition of the circumpolar flow from its normal midwinter state to this preconditioned state are investigated through a combination of observational numerical and theoretical studies. For the 1978–79 winter, this transition occurred during the substantial wavenumber-1 minor warming of January 1979, and the characteristic structure associated with the preconditioned mean zonal flow was established four days after the peak of this war... Abstract In a, series of idealized numerical experiments, Butchart et al (1982) have recently established that the configuration of the mean zonal wind occurring immediately before the wavenumber-2 major stratospheric warming of February 1979 was crucial in subsequently focusing upward propagating planetary wave-activity into the high latitude stratosphere. In this sense, it was concluded that the stratospheric circumpolar flow should evolve to some preconditioned state before a wavenumber-2 major warming could occur. In the present paper, the mechanisms responsible for the transition of the circumpolar flow from its normal midwinter state to this preconditioned state are investigated through a combination of observational numerical and theoretical studies. For the 1978–79 winter, this transition occurred during the substantial wavenumber-1 minor warming of January 1979, and the characteristic structure associated with the preconditioned mean zonal flow was established four days after the peak of this war...