What's Up with the Snowfall Measurements at Alta Guard?

I was recently asked about the large difference in snowfall this past season reported by Alta Guard and Alta Ski Area and thought I would take this opportunity to talk about why they might differ and some of the challenges of observing new snow amounts.  

The Utah Avalanche Center provides a long record of snowfall at Alta Guard at https://utahavalanchecenter.org/alta-monthly-snowfall.  For this past season (November through April), their spreadsheet reports only 432 inches.

In contrast, for the same period, Alta Ski Area reported 606.5" (see https://www.alta.com/weather..note that I've subtracted 21.5" that fell in October from their seasonal total through April).  I am often asked how can there be such a large difference since they are "right across the street from each other." 

Actually, the observing sites are not right across the street from each other.  The Alta-Guard observing site is located just on the north side of SR-210 just down canyon for Our Lady of the Snows and just above the highway at an elevation of about 8660 feet.  Alta typically uses observations collected by their snow-safety at Alta-Collins, which is located in Collins Gulch at an elevation of 9662 feet.  

Map source: CalTopo

Thus, Alta-Collins is 1000 feet higher than Alta Guard.  Mean annual snowfall in the Wasatch Range increases by about 100 inches per 1000 feet, so we might expect these to differ by roughly that amount, but the differential this past season was 174.5 inches, so that doesn't fully explain the difference.

However, it turns out that there is another snowfall observing site at Alta, and that is the volunteer National Weather Service Cooperative observer.  It is my understanding that these observations are collected at the Alta Town Offices that are just up canyon from Alta Guard, as indicated below. 

For the same November-April reporting period, that site reported 498.7" if snow (this data can be obtained from https://xmacis.rcc-acis.org/ or the National Centers for Environmental Information).  There was one missing day in that record (Dec 20), but the ski area did not report snow on that day, so we will assume the seasonal total is complete.  

The difference between this site and Alta Collins is pretty close to what we would expect with that 100" per 1000 feet guideline, but 66.7" higher than reported by Alta Guard only about 500 feet away.  How can we explain this difference?

Here I can only speculate.  First, I do not currently have access to the UDOT observations from Alta-Guard, so I cannot rule out the possibility that there is some missing data that is not being considered or the possibility that there was an error transcribing to to the Utah Avalanche Center site.  I suspect this is not the case, but I cannot rule it out.  

Second, it could simply be that the characteristics at the Alta-Guard site favor snow densification, so that that snow depth from a given water equivalent would be lower. Such characteristics would include, for example, greater wind and sun exposure (and this is consistent with my understanding of the site characteristics).  I do not have access to the water-equivalent observations of snowfall observations from Alta-Guard, but the Alta-Coop site reported 50.68" of water-equivalent precipitation from October 1 through April 30.  It is possible that some of this fell as rain (mainly in October), but that is very close to the 49.8" of maximum snow water equivalent observed by the Atwater SNOTEL site that is just up the hill from the two sites.  Thus, for the sake of argument, I am going to assume that about 50" of water fell as snow this past season at the two sites.  

If that was the case, the mean water content of snow for the season at Alta-Guard was 11.6%, whereas at the Alta-Coop site it was 10.0%.  Such spatial differences in snow water content are not unusual in storms and skiers are well aware that there can be highly localized variations in snow conditions depending on wind and sun exposure.  In small storms, such a contrast is small and likely not to attract much attention.  In a storm that produces 1" of water, 11.6% water content yields 8.6" of snow, whereas 10% yields 10" of snow.  But over a season, it adds up to a more noticeable contrast.  

There are other factors that could be playing a role in the difference between Alta-Guard and the Alta-Coop site, including the frequency of sampling, which I have no information about. The higher frequency that you sample, or simply taking measurements as soon as the snow stops during a storm, yields a greater snow depth than if you do it a low frequency at specific times.  These are the realities of snowfall measurement.  

But there's more, and this is important if you are comparing seasons or looking at trends.  The measurement techniques and site used by UDOT (and the US Forest Service in the more distant past) have changed many times over the years.  Given the large spatial variability of snowfall and the sensitivity to measurement frequency and practices, this makes seasonal comparisons and trends problematic.  

The bottom line here is that one should expect there to be some differences in snowfall reported by Alta Guard and Alta ski area simply due to the elevation difference between their two sites.  One might also expect differences depending on local conditions at the two locations, or measurement practices.  The difference in snowfall between Alta Guard and the Alta Coop site illustrates this well.  I don't consider any of these observations to be "wrong."  The reality is that a single number for new snow depth is going to depend on exactly where you measure it.  As I like to say, "all observations are bad, but some are useful."