Journal Impact Factors (JIFs) appear to increase for the majority of scientific journals. The current analysis was initiated to better define the dynamics of JIFs. Original data from the Journal Citation Reports, from 1997 to 2016, were analysed. The number of citations referring to publications of the previous two years was correlated with the number of articles and the increase in the number of articles. A model was calculated by smoothing the correlation curves. The mean JIF increased from 1.1 to 2.2 almost continuously. The model suggested that the mean JIF will asymptotically reach a maximum value of 2.6. The number of publications has been growing annually by a factor of 1.048. Correlating the overall number of countable citations with the number of published articles revealed a stable relationship of 6.3 citations referring to the previous two years. Validation of the model with a sample of forty-nine journals that have been published since 1961 showed that their recent JIF dynamics are well reflected in the data, but extrapolation of the current dynamics did not reflect the JIFs of these journals in the past. Average JIF is likely to reach a plateau in the future.