OVER-RUNNING TROUGH

An active, mid-latitude frontal system is associated with a marked short-wave trough. The 'active' weather associated with the front lies forward of the trough, driven by the dynamics associated with it. At some stage in its life though, the trough (or a portion of it) will 'relax' (and effectively weaken), allowing the trough to run well ahead of the lower-tropospheric portion of the frontal system - it 'over-runs' the (surface) location of the front, and the activity at that position will decay. Note however that the upper trough will still have 'weather' associated with it - and may be the means of driving an upper/split frontal structure well away from the classical surface front as drawn on conventional analyses.