The present thesis updates the Norwegian and the Shapiro-Keyser model concepts of extratropical cyclogenesis with latest findings in literature, focusing on the last 15 - 20 years. The main properties of the two concepts are then compared to a case study cyclone of each cyclone type. The selection of the case studies is based on polar jet stream configuration, tropopause potential temperature wave and low level frontal shape.
Case study one (starting time: 2017-02-04T12Z), the 'Norwegian' case, begins as a Shapiro-Keyser looking cyclone with a single, merged tropopause fold and T-bone fronts before transitioning to a classical Norwegian cyclone with lambda-shaped fronts, clouds, precipitation and vorticity maxima. It is an explosively deepening cyclone with a central pressure decrease of 42.7 hPa in the 24 hours before reaching peak intensity of 933.6 hPa. The reason for the cyclone transition is the high-amplitude downstream ridge that shortens the wavelength of the cyclone and causes it to move northwards in an area of large-scale diffluence.
Case study two, the 'Shapiro-Keyser' case, starts at 2016-10-27T00Z and does not deviate from the conceptual model at all. It evolves with a dry airstream, also deepens explosively (31.1 hPa pressure drop in 24 hours) and the fronts, vorticity, clouds and precipitation exhibit a T-bone structure with a frontal fracture at the northern end of the cold front. At the end of the life cycle this cyclone undergoes secondary development as an occluded Norwegian cyclone. This is partly due to the meridional low and the subsequent interaction with the landmasses of Greenland and Iceland. Secondary developments like this are quite common and have been observed by weather forecasters for years.