An experiment, aimed at forecasting the formation and intensification of extra-tropical cyclones, was conducted from 4 January 1954 to 20 March 1954. The working hypothesis to be tested was formulated as follows: cyclonic development at sea level occurs when and where an area of positive vorticity advection in the upper troposphere becomes super-imposed upon a frontal zone at sea level. The experiment, which was conducted in connection with the routine operation of the U. S. Weather Bureau's District Forecast Center in Chicago, resulted in 97 verifiable forecasts. The verification indicated that the working hypothesis is useful.