A 30% chance of rain/thunderstorms is more likely to mean that the storms have a 100% chance of occuring over 30% of the area, and that area is usually at the higher elevations; so you will probably get thunderstorms on Mount Whitney on a day when there is a 30% chance of them. The forecast was not dead wrong.
More info:
http://www.srh.noaa.gov/ffc/?n=pop
