The development of Atlantic subtropical cyclones is examined in terms of the potential vorticity (PV) anomalies, associated with cold upper cut-off lows and lower troposphere circulations. It is proposed that mixed baroclinic/barotropic instability, and then PV superposition may provide a conceptual reasoning for the cyclogenesis (and subsequent tropical cyclogenesis) of warm season subtropical storms, such as Karen during 2001 and Michael in 2000. At the relevant stages in their life cycles, these storms may be defined as Type 2 (cyclogenesis source) subtropical cyclones, as described by Simpson (1952). Results from a two-layer model are presented, in support of this theory.