Why There's a Huge Snowfall Contrast from Alta to Park City

The snowfall contrast between Little Cottonwood Canyon and Park City Mountain Resort over the past 24 hours is quite dramatic.  

In upper Little Cottonwood Canyon, Alta reported 30 inches of snow. 

In upper Big Cottonwood Canyon, Brighton reported 22 inches of snow.

On the Park City Ridgeline, Park City Mountain Resort reported only 12 inches.  

The distance between Alta the top of the Jupiter chairlift at Park City Mountain Resort is less than 5 miles!  That's a huge contrast in snowfall.  How can this happen?

One of my late-career dreams is to have the University of Wyoming King Air research aircraft flying through a storm like this one with it's world-class cloud radar.  This instrument takes cat scans in storms.  One thing I would like to have it do is fly directly down the northwesterly flow and examine the storm structure from northwest to southeast.  Here's why.

We often generalize the western side of the Wasatch Range as the "windward" and wet side and the eastern side as the "leeward" and dry side.  That, however, is an oversimplification.  In reality, the terrain felt by the flow varies depending on wind direction and because the Wasatch range isn't linear, but contains complex ridges, canyons, and inflections.

For example, the late-afternoon sounding from yesterday showed that the 700-mb (crest level or 10,000 foot) flow was from 315˚.  If you are in Little Cottonwood Canyon or the high terrain surrounding it, flow from this direction (i.e., along the thick black line below with the topography along that line in profile at the bottom) moves across the relatively low Salt Lake Valley and then is forced rise rapidly over the Cottonwood Ridge and Alpine Ridge.  These two ridges are over 11,000 feet high and represent the highest terrain in the central Wasatch.  During unstable flow, this results in persistent initiation of clouds and precipitation just upstream and over Little Cottonwood Canyon.  

Source: caltopo.com

As you move northeastward to the Park City Ridgeline, the situation changes.  Not only is the Park City Ridgeline lower (highest peaks 10,000 feet, it is also oriented parallel to the flow.  In addition, because the Wasatch near North Salt Lake City and Bountiful just westward, the flow encounters mountains well upstream of the Park City Ridgeline.  Instead of rapid ascent over about 6,000 vertical feet, the flow ascends more gradually, encountering small ridges along the way.  

This is simply not a great situation for heavy snowfall on the Park City Ridgeline as the terrain-forced ascent is less intense.  Indeed, snowfall reports from Bountiful are in the 10-14 inch range, comparable to that reported by Park City Mountain Resort.  

There are probably some other factors involved as well, but I think these differences in the shape and scale of the terrain felt by the flow from the northwest are a contributor.  Perhaps more on other factors in future posts, but I'm going to have to get some real work done soon.