Forecasting in Tableau does not generate data and the alert mechanism relies on data. You have a few options. You might vote on this idea - https://community.tableau.com/ideas/8541 and/or create a new idea that captures the specific use case of generating actionable data from forecasting in Tableau. The other option is to investigate utilizing R to generate the forecast and then act on the calculation from R as an alert.
I am moving this thread to the Server Administration area of the forums for more feedback/discussion.