If the National Weather Service issues a Winter Storm Warning, it's a good idea to stay off the roads. However, a Winter Weather Watch means that there is potential for a winter storm in the near future, and the weather service will continue to monitor the forecast closely.

A warning is issued when forecaster confidence is high that a storm will occur; a warning means it is the time to prepare. An advisory is similar to a warning, but impacts will not be quite as bad.

Projected snow totals play a big role in deciding whether an advisory or a warning is issued, but a storm's impacts are just as important, if not more so. For instance, is the snow going to fall during the morning or evening commute? Will there be a lot of ice? Will visibility be low? 

Above 7000' a warning is issued if roughly a foot of snow falls in 12 hours, or 18 inches in 24 hours. In the valley, it's about four inches in 12 hours. An advisory would be about half of that. 

A Blizzard Warning is issued when sustained winds or frequent gusts are at least thirty five miles per hour, lowering visibility to less than a quarter mile for three or more hours. So it's actually more about the wind than the amount of snow.