Avman,
From a purely physical point of view, I should imagine it would be a pretty smooth ride, albeit noticeable. My reasoning behind this is to think of an aerofoil moving through the air - consider the surface of the aerofoil and the boundary layer of air to be the jetstream core. Now think how this affects air for some distance away from the aerofoil itself, right up until a point of sufficient distance from the aerofoil that it is no longer affected. So it would seem the logical answer to me is that it would be a gradual change.
Of course I stand to be corrected
Jack