  We present a classical model for high harmonic generation during the
propagation of an intense laser pulse through an atomic gas. Numerical
simulations of the model show excellent quantitative agreement with the
corresponding quantum model for the blueshift and intensity reduction of the
propagating laser pulse over experimentally realistic propagation distances. We
observe a significant extension of the high-harmonic cutoff due to propagation
effects. Phase-space analysis of our classical model uncovers the mechanism
behind this extension.
