Are Major Winter Storms Increasing in Frequency and Intensity?

After major winter storm cycles, especially those that lead to long Interlodge periods in Little Cottonwood Canyon, I get a lot of questions about whether or not snowstorms are getting bigger due to climate change.  Many people assume this is the case, but it turns out to be a difficult question to answer, especially if you are talking about a specific location like Little Cottonwood Canyon.  

Let's talk first about what we know.  Globally averaged temperatures have increased about 1˚C and Utah has warmed at almost double that rate.  Recent studies indicate that most if not all of the warming in recent decades is due to human activities, especially greenhouse gas emissions.  

Precipitation is a messier variable, however, as it exhibits greater regional variability than temperature.  It is also not sampled as well, with fewer stations and often lower quality measurements.  Finally, there are two questions that one might ask.  First, is the frequency of extreme precipitation events changing (i.e., are they becoming more or less common). Second, is the size (or intensity) of extreme precipitation events changing (i.e., getting bigger or smaller).  

The answers to these questions vary regionally.  It is likely that there are more land areas where the frequency of heavy precipitation events has increased rather than decreased and it is likely that there has also been intensification of the heaviest precipitation events.  For example, the total annual precipitation produced by the largest (top 1%) precipitation days has increased throughout much of contiguous United States, but has remained relatively steady over the southwest.  

Source: https://www.globalchange.gov/browse/indicators/heavy-precipitation

Major winter storms are even more difficult to assess and project trends for and they have an additional confounding factor because in a warming climate, some precipitation events will feature rain instead of snow.  The US National Climate Assessment released in 2017 concluded the following:

“Projections of winter storm frequency and intensity over the United States vary from increasing to decreasing depending on region, but model agreement is poor and confidence is low.”

I have never felt very confident saying that winter storm intensity is increasing in northern Utah for several reasons.  First, snowfall records are much more limited than temperature records, both in their historical coverage and their spatial density.  This makes detecting trends in rare events even more difficult.  Second, our computer models struggle more with precipitation than temperature and, if we're talking about what happens in Little Cottonwood Canyon, don't even account for topographic effects or lake influences.  Additionally, those models don't exhibit a great deal of agreement in how annual average precipitation will change in northern Utah, let alone extreme events.  

Some people may say that we should be able to try theoretical thinking toward this problem.  The challenge with that is many factors affect winter storm intensity.  In a warming climate, the water holding capacity of the atmosphere increases, so storm intensity should increase.  If we are talking about global statistics, that may hold, but regionally that effect could be countered by other changes.  For example, if the storm track shifts northward or ridging becomes more common, we may see fewer storms, so in our area, the frequency of big events goes down.  At some elevations, the "big" storm in a warming climate could be a rainstorm instead of a snowstorm.  Tough luck there. 

So, I can't confidently determine if major winter storms are increasing in frequency or intensity in northern Utah or Little Cottonwood Canyon.  I'm comfortable saying that we don't know, but that we have good evidence that other things are happening to our snow climate that should concern us, especially in the lower elevations where we are seeing a decrease in the fraction of cool-season precipitation that falls as snow, more frequent mid-season snow-loss events, and a decrease in the amount of wintertime precipitation retained in the snowpack at the end of the snow accumulation season.  These trends are less dramatic at upper elevations, but will amplify in the coming decades, especially if greenhouse gas emissions continue.