Meteorologists typically refer to the ratio of mountain to lowland precipitation as the orographic ratio. In a storm in which the mountain and lowland precipitation is the same, the orographic ratio is one. If the mountain precipitation is four times the lowland precipitation, the orographic ratio is four. In an upside down storm, the orographic ratio is less than one.
On average, the orographic ratio between Alta and Salt Lake City during January is nearly 5.5. This is the highest average orographic ratio of any month during the year and illustrates that on average January features the greatest enhancement of precipitation by the Wasatch Range.
Most research suggest that the smallest orographic ratios along the Wasatch Front occur during frontal passages. In part, this is because the front provides most of the lift for precipitation formation and the mountains play a less prominent role. In some instances, the front can be fairly shallow and the storm dynamics are actually better over the valleys than over the mountains.
For example, during the November 2001 Hundred Inch Storm, there were two storm periods. In the second, the orographic ratio was smallest (and equal to 1.3) during the frontal passage, indicated by the frontal storm stage below. Note how Alta received only 1.3 times as much precipitation as Salt Lake during the frontal passage.
That being said, these are generalizations and there are a lot of factors that affect precipitation. Producing models that can better predict the distribution of precipitation is an area of ongoing research and one in which we sorely need advances to help us anticipate everything from deep powder to commuter disruptions.