Great question. You are trying to do what/if scenario work with Tableau which is really cool. I can only think to create a new measure using a parameter to cover the months in the future that are plus/minus the most recent month of data and then add a forecast from this new measure. A workbook would help here. What you can't do is have a forecast that overlaps data in the view so if there is data for September, 2016 (future), then no forecast will be built for that same date value.
If you are going to get really detailed about your forecasting, you might consider integrating Tableau with R to do statistical analysis. There are a lot of online resources to assist with that effort.
I hope that helps.