I like what you're trying to accomplish, but you are missing a very helpful piece of information in your analysis. I'm sure you noticed how poor of a predictor rain fall is in your assessment of how much water will be added to Lake Oroville. This is largely because Oroville is filled from Sierra Nevada runoff. If a particular storm system is warm (as was the case with the latest one), it will mostly rain in the Sierras. This has a twofold effect: one, the precipitation doesn't stay put at higher elevations (as snow does), and two, it melts the existing snow, causing it to also be added to the downstream water accumulation. So a key part of forecasting is not only to look at expected precipitation, but also the expected snow levels.