This type of cyclogenesis involves the development of a cyclone out of a cloud system originating upstream of the trough axis.

If the cold air feature associated with Jet J2 rounds the base of the upper trough and approaches within 300km of the polar front cloud band, the two systems may merge. The resulting cloud formation resembles the classical occlusion of a mature depression, without the occlusion process .

Split Flow cyclogenesis (originally proposed by Weldon (1975)) is the rarest form of cyclone development, as it often requires local topography to trigger a double jet with a split flow configuration - i.e. it occurs to the east of mountain ranges .

Meridional trough cyclogenesis occurs when a synoptic scale trough is present in the upper atmosphere and the airflow around the trough is symmetrical - i.e. there is no appreciable diffluence or confluence present at the entrance or exits of the trough..

An upper trough with zonal flow upstream of its axis will induce cyclogenesis if there is marked diffluent flow in its downstream region. The trigger for cyclogenesis is the ascent in the left exit of the jet streak moving through the diffluent downstream region of the trough. This ascent manifests itself as the emergence of the warm converyor and produces a mid-level cloud extrusion .

Induced-Wave cyclogenesis involves a polar trough approaching to within 300km of the polar front, but without any merging of the two systems.

An upper trough with confluent flow downstream of the trough axis may develop a mid-level cloud extrusion on the poleward side of the polar front cloud band.