Wednesday, October 17, 2018

Impacts of a Shrinking Great Salt Lake on Wasatch Snowfall

How much does a shrinking Great Salt Lake affect Wasatch snowfall?  It is a question that I am frequently asked and one that of course has a complex answer.

To begin, we should instead ask the question, how much of the snowfall in the Wasatch Range is produced by lake-effect precipitation?  Conventional wisdom might suggest a lot, but the actual numbers are probably lower than most people think.

Kristen Schepel (née Yeager), Trevor Alcott, and I set out to answer this question several years ago as it hadn't been adequately answered previously.  The effort involved combing through a gazillion radar images over 12 cool seasons (mid September to mid May), identifying lake-effect periods, and figuring out how much precipitation at observing sites in northern Utah is produced during those periods. 

The results are below and indicate that in an average cool season, lake-effect periods (LEPs) produce about 60 mm (2.4 inches) of water equivalent at the Dry Fork SNOTEL (DRFU1) in the Oquirrh Mountains and the Snowbird SNOTEL (SBDU1) in the Wasatch Mountains [Left Hand Figure].  Of the observing sites we examined, these sites receive the most lake-effect period precipitation.  At the Dry Fork SNOTEL, this represents about 8.4% of the total cool-season precipitation.  At Snowbird, about 5.1%.  Not surprisingly the area south (Oquirrh Mountains) and southeast (Salt Lake Valley and central Wasatch) receives the most precipitation during lake-effect periods, however, this represents less than 6% of the total cool-season precipitation at all sites except those in the Oquirrh Mountains.  Note that estimates at some valley sites, such as Cottonwood Weir at the base of Big Cottonwood Canyon, are probably too low due to undercatch of lake-effect snowfall by gauges that are not shielded from wind effects. 

Source: Yeager et al. (2013)
It is worth noting a few caveats about this analysis.  First, we did not attempt to discriminate between lake-effect and non-lake-effect precipitation during lake-effect periods, and the two do occur in concert at times.  This would contribute to some overestimate of the total lake-effect precipitation.  On the other hand, the lake may at times contribute to some enhancement of precipitation features that don't necessarily look like lake-effect features, and we did not consider such effects. 

So, let's put those numbers into some perspective for skiers, beginning with Snowbird as a proxy for upper Little Cottonwood Canyon, which on average probably receives the most lake-effect precipitation.  If the lake were to disappear, and lake-effect ended, the average impact on cool-season snowfall would probably be a reduction of about 5% (lake-levels during the period we examined varied from above average to below average, so I'm assuming the lake-effect snowfall is generally representative of the long-term mean).  That equates to about 25 inches of snow assuming a 500 inch mean.  Elsewhere, the reduction would be smaller.  For example, Snowbasin and Park City/Deer Valley receive less lake effect, so the loss of the Great Salt Lake would have an even smaller impact.

For further perspective, we might also ask the question, with global warming, how much of the precipitation that previously fell as snow would instead fall as rain.  Estimates suggest that 4ºC of warming (about 7.5˚F), as is presently projected with continued greenhouse gas emissions, would result in a 20% decrease of snowfall at Snowbird (mid mountain estimate) due to more precipitation falling as rain (declines are greater at lower elevations).  The direct impacts of global warming on snowfall and snowpack remain the biggest threat to the Greatest Snow on Earth.  Changes to the Great Salt Lake are a secondary effect. 

However, there is something that I have swept under the rug in the analysis above and that is the importance of the seasonality of lake effect.  If one looks at Snowbird, for example, the peak in event frequency and amount of precipitation (snow water equivalent) produced by those events is in November. 

Seasonality of lake effect at Snowbird. Source: Steenburgh (2014)
The 1.2 inches of average SWE produced in October and November is quite significant when one considers how important just a little snowfall is for starting the ski season and for resort operations during Thanksgiving and even the Christmas Holiday.  On the other hand, that November total is strongly affected by two monstrous lake-effect events that occurred over Thanksgiving 2001 (and contributed to the famed "Hundred Inch Storm").  Basically, there are some years where we get a major lake-effect event and in those years, lake-effect makes an important contribution to the early season snowpack. 

To conclude, the contribution of the Great Salt Lake for Wasatch snowfall is frequently overstated, but that is not to say it is unimportant.  Events in the fall, especially larger ones that happen episodically (not necessarily every year), can be important for early season snowfall and the timely start of the ski season.  However, the 800-pound gorilla in the zoo is really global warming, and its direct impacts on Wasatch snowpack and snowfall remain the biggest threat to the Greatest Snow on Earth.

That being said, the Great Salt Lake is an absolute treasure, and it would be a damn shame if it, like other terminal lakes around the world, was condemned to a noxious dust-bowl future. 

Near Spiral Jetty, 15 June 2013


  1. There is a further effect that is non-quantifiable: the psychological effect on skiers, seeing snow on the ground at home and heading for the slopes. It's real, and a business success factor. Less snow in the valley = less skiers on the slopes.

  2. But is there a correlation between the lake surface area and annual lake effect snow accumulation or number of lake effect periods?

    1. Not a useful one. The amount of lake effect is more strongly influenced by the weather pattern in any given winter.

  3. What is the cooling effect from the GSL? I.E. does it actually also cool the Wasatch front in summer, when there is a westerly wind?

    1. Yes it does, in a variety of wind regimes. That effect is strongest over the lake and in the northern Tooele Valley. It is weaker along the Wasatch Front and in the Salt Lake Valley, although there are times when it is apparent.