May Snow in Context

Yesterday, two skiers died tragically in an avalanche somewhere near Big Willow Cirque on Lone Peak (details still forthcoming).  This after a storm cycle in which Alta Ski Area reported 42" of snow and even the benches received substantial accumulations in some areas.  How unusual is such May snow?

This is a hard question to answer concretely mainly because the available records at bench and upper elevations are spotty, inconsistent, and limited.  For bench level, I will fall back to my usual standby, the Bountiful Bench cooperative observer site, which is at an elevation of 4950 feet.  This is a bit lower than perhaps one would like, but it has continuous records back to 1975 with no missing data.

First lets look at snowfall during May. This station has observed 1" or more of snow in 20 out of the 50 Mays on record, with a mximum of 27.9" in May 1975.  Impressive!  Second is 1983 with 15.6", followed by this May with 10.2", although it should be noted that this May is not over yet.   

Source: https://xmacis.rcc-acis.org/

For this location, this May was the snowiest since 1983, although it should be noted that this May is not over yet.  

If one looks at the largest 3-day May accumulations, they have all occurred in 1975, 1983, or 2024 (note that some of these overlap).  

https://xmacis.rcc-acis.org/

Going down to the largest 2-day May accumulations, events in 1991 and 1979 also sneak in there, with this May coming in at #5.  

https://xmacis.rcc-acis.org/

And finally for a single day, the 5.7" that fell on May 6 of this year and 4.5" on May 5th are good for #5 and #7 since 1975.  

So, at least at this location, I would describe this latest May storm cycle as unusual but not unprecedented.  One can find snowier 1-3 day periods in at least two other periods, although it has been over 40 years since that happened. 

The Bountiful Bench site reported 10.2", but in Public Information Statements issued by the National Weather Service, I saw as much as 11.5" reported from Bountiful at 5117 ft at 7 AM on Tuesday and 12" at the City Creek Canyon Water Plant at 5340 ft at 4 PM Monday.  I also saw a report of 15" at 5600 feet in Emigration.  It's harder to place these in context, but I suspect the general view that it's unusual, hasn't happened recently, but is not unprecedented probably holds, but that's admittedly a conjecture.  

At Alta, the cooperative observer by the municipal offices has recorded 40.9" of snow so far this month.  Their records are not as complete, but there are 12 prior Mays that top that.  The 18.3" that fell this May 6th was the 3rd highest for a single day in May (topped by May 9, 1986 with 20.5" and May 11, 2000 with 20") and the 30.1" that fell on May 6 and 7 was also good for 3rd highest.  For a five day period, which one can find big cycles in May 1986 and 1993 that were actually quite a bit bigger than this May.  The maximum 5-day accumulations in those months were 67.5" and 66.0", respectively, compared to 50.1" this May.  

https://xmacis.rcc-acis.org/

Given that there's spottier records for Alta, it's a bit harder to convert that into a frequency or return period.  My take is what has happened this May at upper elevations is certainly not unprecedented and I'm not sure I would call it unusual like the bench snowfall. If we had more continuous records we might find a storm like this week's happening every 5-10 years at this elevation in May.  Back to 1971, even with substantial missing data, the Alta Coop site has observed at least 40" of snow in May 13 times, which is roughly once every 4 years, but the return period for more intense, shorter period snowfall would probably be longer.  

That's enough handwaving for today.