THE ROLE OF A DEVELOPING UPPER DIFFLUENT TROUGH IN SURFACE CYCLOGENESIS OVER CENTRAL MEDITERRANEAN

An attempt is made to investigate the role of the jet streaks upstream an upper tropospheric diffluent trough in its evolution. A proof of t he relationship that controls the rate of change of the mean absolute vorticity in the region of the diffluent trough is provided. It is dem onstrated that for certain value of the rate of the increasing mean re gional vorticity, a cut-off low can develop in the upper troposphere, which is likely to trigger the initiation of surface cyclogenesis, und er favourable lower tropospheric conditions. Therefore, it seems that this approach offers to the operational meteorologists a simple tool t o evaluate the numerical weather prediction of a cyclonic development in southern Europe and to decide for its uncritical accept or rejectio n when the initial atmospheric conditions in northwest Europe are simi lar to those described above. This diagnostic tool is then applied to the case of cyclogenesis that occurred on the 29(th) January 1994 in G reece, in order to demonstrate its practical significance.