In the United States, some severe convective events are frequently associated with east-west oriented quasi-stationary frontal zones. It is observed that in Europe undulating frontal zones with minor low pressure centres are often involved. Sometimes severe convection develops along the cold or warm fronts, but often convergence lines on the warm side of the frontal zone are the places where most convection occurs. Their nature is not yet well understood. In Europe, severe convection on the cold side of the frontal zone is infrequent due to a lack of instability in the cold air. The orientation of the frontal zone in a typical severe weather outbreak in Europe seems to be somewhat dependent on the geographical location.