Currently the forecast is based on the sales during the timeframe that you set.

We are working to add the seasonal factor in forecast results soon.

However, you can use this app for getting seasonal forecast by selecting the time frame of last year, to predict the seasonal sales this year.

App has your historical orders of last 12 months.

If you chose say the month of November of last year and use that to forecast for the month of November of this year.

You can use 'custom range' option to select the old timeframe. Export the forecast to csv file, then add a guesstimated growth % of your own, to get a more accurate forecast.