In early September 1999,a tropical storm(WENDY)landed,moved northeast,then interacted with the trough in mid latitudes and finally, produced tornado in the southeastern Shanghai. Combined satellite cloud pictures with physical quantities, it is found that interaction of systems between middle and low latitude made WENDY inverted trough maintenance and development, also play an important role for WENDY moving to north. When WENDY connected with westerly system , good environment of higher level divergence and lower level convergence strengthened the development of convective cloud. East moving of the mesoscale dry surge intensified the horizontal wind shear, at last caused the tornado.