Blown Weather Forecasts - Why/How?

Those of us in Tahoe/Mammoth might have noticed a slight discrepancy in the forecasted weather for Saturday night into Sunday vs. the actual weather.

Being a scientist myself, I will be one of the first to recognize the complexity of foreacasting weather for a mountainous region such as the one I live in. I am curious, however (WITHOUT POINTING FINGERS OR PLAYING THE BLAME GAME) about what meteorological/atmospheric changes can so drastically alter the observed weather in such a small amount of time. Example - it seemd to my uneducated eye that there were lots of clouds and moisture poised off the CA coast Friday and Saturday night, set to slam into the mountains, pick up some moisture from the central valley, and do it's usual dumpage of snow in the Sierra. Alas, Sunday dawned stormless and dry. Now, the weather guys are calling for a huge storm tonight into tomorrow night?

Did the jet stream change course? Did the moisture disappear? Did the storm stall? What makes an 80% chance of forecasted snow turn into bluebird cloudless day in 12 hours?