Apparently the lumber price is a leading indicator and a proxy for the housing market. (Zero Hedge)
If the lumber price goes up, 2 months later, the housing market goes up. The same happens the other way round.
So it's very important to watch the lumber price, which can be found here.
And even more important, if this correlation is real, then you can just predict the market and earn money by betting on the direction of the market.