Snowbird SNOTEL Measurement Oddities

Those of you keeping score at home know that through last week, the Snowbird SNOTEL had still not eclipsed the peak snowpack water equivalents reached in the 2005 and 2011 water years.  

However, things changed over the weekend.  

On April 26th, the snowpack water equivalent was 72.3", the highest of this season, but a shade lower than the 75.1" reached on two dates in 2001 and the 74.6" peak in 2005.  However, from April 26th to 30th, the snowpack water equivalent rocketed upwards to 76.2", a new record.  

Source: NRCS

How did this happen?  Certainly not from snowfall, as the last storm was on April 25th and would have been accounted for by the measurement on the 26th.  The 26th was a spectacular day with great skiing in the wake of that storm and on the 27th the site recorded 72.0", just a shade lower than the 26th.  Then from the 27th to the 28th the measurement jumped to 75.2", despite there being no significant precipitation.  The 27th was a very windy day and perhaps the wind deposited snow over the pillow.  However, it went up even further to a peak of 76.2" on April 30.  How could this happen?  

SNOTEL stations measure snowpack water equivalent using a snow pillow with a pressure transducer.  The snowpack weight of the snowpack presses down on the pillow, resulting in pressure reading that can be converted to a water equivalent.  

Source: NRCS

The pillow, however, simply measures pressure.  It doesn't say anything about where that pressure came from.  Increases could be due to snowfall adding weight to the snowpack, but also other factors like wind transport adding to the snowpack over the pillow, or water moving horizontally through the snowpack during melt periods. A strong layer in the snowpack can also result in bridging, with the snow above the bridge not fully contributing to the weight of the snow.  If the bridge weakens, then there can be an increase in measured water equivalent.  

Snowbird isn't the only location showing this behavior.  Lookout Peak jumped 2" from April 26 to April 28 before dropping a bit through today.  That increase, not associated with precipitation, also pushed Lookout Peak to a record snowpack water equivalent.  

Source: NRCS

Those are a couple of the curious measurement oddities evident over the past couple of days and a reminder that SNOTEL time series are not just a reflection of precipitation or melt.  

That said, we can also have a look at stations where the snowpack has ripened (i.e., warmed to the melting point through the entire depth of the snowpack) and net melting are occurring.  A good one is Parleys's Summit.  On April 28th, snowpack water equivalent was 32.9" and in 3 days it has dropped over 5" to 27.7".  The decrease yesterday was 2.2".  Pretty impressive for the last day of April. 

Source: NRCS

With a warm night last night, a decent amount of sunshine, and another day near or above record highs,  expect the impressive snowmelt to continue at Parley's and other areas where the snowpack is ripe.  

Addendum 2 May 2023:

NRCS appears to have done some quality control of the Snowbird data. When I pulled it up this morning, the big increase was reduced and values were below the 2011 water year maximum.  

Source: NRCS