When the cold front overtakes the slow moving warm front, the warm air is forced upward. The meeting of the two fronts creates an occluded front. Under normal conditions, in the later stages of occlusion the cyclone weakens and ultimately dissipates.
Image Source: nasa.gov/images/content/514175main_Yasi-MODIS-LARGE.jpg
But sometimes, during the late maturing stage of a cyclone, a new low develops on the equator-ward margin of the original cyclone. This usually takes place in southeastern or south-western part of the polar front.
Thus, a secondary cyclone is formed which passes through different stages of its life cycle and matures very rapidly. It may follow the track of primary cyclone or may move along a new path.
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The Norwegian meteorologists also found that an extratropical cyclone never appears alone. It is usually followed by three or four cyclones forming a series. The primary or the leading cyclone gets occluded, while the new ones originate on the trailing front and are in an incipient stage.
In the rear of the last member of the cyclone family there is an outbreak of polar air which builds up an anticyclone.
It is to be noted that the original cyclone would be in high latitudes, and each secondary cyclone would follow progressively a more southerly path. Cyclone families frequent the oceans in a larger number. But on the continental areas they are not so regular.
According to Petterssen, the secondary cyclones and members of the cyclone family play a far more important role than the individual cyclones and anticyclones in bringing about latitudinal head and moisture exchange. It is through these horizontal exchanges that the latitudinal heat balance is maintained.