Cold-Front wave

A secondary low pressure system forming on an extended cold front, where the thermal contrast across the front (in the troposphere) is large, and the upper pattern is conducive to falling pressure at the surface. The wave can move quite rapidly (in the direction of the general upper driving flow), and will lead to a hesitation in the clearance of the main cold front at the surface, or its return to areas that previously experienced a clearance. Not all such waves develop closed-low characteristics; some will just 'run' quickly along the length of the trailing cold front with little development, other than enhancing rainfall. Because of the small-scale of the initial development, NWP models don't always place and forecast these correctly. (See also warm-front wave.)