Seasonal Forecasting Trend

The line below the data takes the ratio of the month to the yearly average excluding the month. (Literature says to compute two averages, and then average them: six months before plus five months after, and five months before plus six months after. The filter I use does that in simpler fashion.)

The line below that normalizes the average about 1, i.e., the average of the second row is exactly one. (It would be better if there were more than one whole year of data).

The forecast uses an underlying exponential fit (the GROWTH function) to forecast the growth, corrected by the seasonal variation computed above.

I think the whole thing would be accurate if you had an integer number of years (several, four or five) over which to compute the seasonal variation. As it is, the seasonal variation itself affects the exponential fit -- but again, there ain't much seasonal variation.

If you work for the Fed, please consult an expert