A method is devised (based on the theory of Bjerknes and Holmboe on the deepening of cyclones), for the forecasting of the deepening of tropical cyclones. It is shown that the distribution of divergence around an easterlies trough (“easterly wave”), as revealed through consideration of the vorticity equation agrees with this application to the deepening of tropical cyclones. The conclusion is reached that an estward tilt of the pressure, or streamline, axis of a trough or developing tropical disturbance is most favorable for deepening; whereas, in deep easterlies, a westward tilt of the axis would result in filling and dissipation. A technique is also suggested for a quick qualitative evaluation of the direction of slope of the axis of such a pressure or streamline system.
* Department of Weather, Chanute Field, Ill.