The interaction between diffluence and potential vorticity of the midtropospheric flow prior to the onset of selected blocking episodes over the Southern Hemisphere is investigated. A diagnostic equation for geostrophic u-component wind tendency, in which deformation appears as part of a forcing function, is applied to examine the interaction between diffluence and potential vorticity, as well as interaction between deformation field and the wind field during block development. Results indicate that the increasing geostrophic easterlies in the mid-latitude midtroposphere during block onsets are forced by the poleward increasing anticyclonic vorticity advection, which has a primary influence on the movement and propagation of the wave system. Also the horizontal gradient of differential thermal advection and deformation terms play the secondary, but important roles in the wave development. In current diagnostic experiements, "extended" wind tendency equation is being applied to the blocking cases. Here the geostrophic wind tendency is replaced by the observed wind tendency, which is forced by nongeostrophic processes including three dimensional varying static stability. The quality of the two wind tendency models(quasi-geostrophic and extended) is determined by comparison with corresponding analyses using three methods: area-mean wind tendencies are calculated over the whole computational domain to compare the magnitude of the model and observed wind tendencies; linear correlation coefficients are calculated to measure the degree of pattern agreement between the model and analyzed fields; maps of wind tendency fields are compared in order to further qualitatively identify their similarities and differences. Results will be reported.