Thanks, WebHubTel. I agree it is odd they don't point out the strong periodicity in the historical record.

But I haven't seen you actually demonstrate your ability to extraprolate the QBO. Its obvious from your graph this is possible to some extent, but how much? If you feed half of the data (1953-1983 say) into FeedFormula and then extrapolate to 2013, how accurately can you do this? I would want to do lots of tests like this before extrapolating into the unknown.

Your formula looks odd to me because all the cos terms are equal to 1 when Y=0. I can see that the data is at a peak near here, but you can't know this is exactly true, and it would be quite wrong for other time periods.