The filament due to the self-guided propagation of an infrared femtosecond laser pulse in atmospheric-pressure air is used to trigger and guide an electric discharge. The long low density plasma channel due to the filament is first heated by the Joule effect during an initial transient plasma stage. The heated channel of recombined gas then hydrodynamically expands radially. The onset of a discharge starts when the density depression on axis reaches the threshold discharge value. This model is supported by detailed experimental and numerical analysis.