Most weather models will predict a precipitation amount in terms of inches of water (doesn't matter what the precipitation type is). From there, the snow:water ratio tells you how much snow you will get out of it (and that ratio is based on temperature aloft, humidity, and many other factors). I seem to recall someone saying they like at least a 15:1 ratio to call it powder, or greater than 20:1 for the high-quality Utah stuff.
This page tracks snowfall history at Alta in terms of inches of snow and inches of water equivalent:http://alta.com/pages/snowhistory.php
A lot of it's in the 10:1 to 15:1 range, but there are a few 20:1 events this season that were probably wonderful to ski in.
If you want to know more, Google "Kuchera algorithm snow" for a model that can predict the snow:water ratio. It will show all the input variables.