The USBR gives their forcasts for the rivers and reseviors in NorCal.
The 90% forecast means that 90% of the time the water level will probably be better than shown and 10% of the time it will be worse than predicted.
see:
http://www.usbr.gov/mp/cvo/data/jan90B2.pdf for the 90% prediction.
I have to say, that if my local lake meets forcasts, it will be just 16 vertical feet below the top of the dam at the peak in May. Yipee!