It is hypothesized that the seasonal variation of the background flow, episodic non-modal forcing and nonlinear dynamics of the disturbed flow are important for the excitation of intra-seasonal variability in the extratropics. In contrast, it is not necessary to have linear modal instability. This hypothesis is substantiated with an analysis of a forced dissipative quasi-geostrophic model. The episodic local forcing is intended to mimic the influence of synoptic scale disturbances of baroclinic origin. The model response is diagnosed in terms of potential vorticity (PV) dynamics, the EOF°¦s of the fluctuations, the related principal components and the corresponding energetics. The evolution of the response controlled by PV advection alone is found much less chaotic than the counterpart with additional PV fluctuations associated with the background seasonal flow.