The counter-intuitive statistical characteristic that the storm track over N. Pacific has distinctly weaker intensity in mid-winter than in early- or late- winter remains largely unexplained. It is hypothesized that the phenomenon is a dynamical consequence of the change in the isentropic potential vorticity gradient across the tropopause over N. Pacific. Such change gives rise to an increase in the barotropic-governor-effect on local baroclinic instability. The observational basis of this hypothesis will be presented. The hypothesis is substantiated by an instability analysis of a quasi-geostrophic model constructed under the guidance of the dynamical considerations alluded to above. A series of experiments illustrate that such background PV structure would lead to a reduction of the local growth rate, a shift of maximum eddy amplitude downstream of the jet core and a modulation of the shape and amplitude of the traveling wave packets. The overall dynamical process may be succinctly conceptualized as wave-packet-resonance associated with the barotropic and baroclinic components of an unstable wave packet. Additional insight of this process is deduced from the characteristics of their local energetics.