Jamie,

The SOI usually goes way negative if there is/are developing cyclone(s) in the WPAC. A negative SOI over the long haul usually portends El Nino. It can signify blocking or amplification in the US 15ish days down the road. This can help with the idea of whether or not the eastern US will show trof conditions or just a flatter flow across. Depending on other variables, this can help inidicate whether or not the Gulf or East Coast is at risk at a given time. It doesn't portend storms on this side of the ocean, but it can help with paths. The NAO breaking negative is one of the better signals (in non El Nino years) of a coming burst of tropical activity.

Steve