The ratio of snow to water can vary a great deal depending on the vertical profiles of temperature and moisture, and how they change during a storm. Typically 1 inch of rain is equal to 1 foot of snow, a 12-1 ratio. Depending on the temperature and moisture profiles of the snow growth region of the atmosphere and the origin area of the storm system, that ratio can go higher, say 20-1, which would be 20 inches, or lower, say 6-1 or so. 12-1 is most forecasters starting point, and if you go to your local NWS page and read the area forecast discussion, they usually tell what ratio they believe a system will have as it moves through your area.