Monday, March 30, 2020

Snowpack Status

April 1st is commonly used as the day when the snowpack water equivalent (SWE) is at it's peak in the mountains of the western United States, although in reality it varies depending on aspect and elevation.  For example, peak snowpack on north aspects at 9500 feet isn't reached until late April.  Nevertheless, we'll take a look at where things stand today, a couple of days early, as an indicator of how the snow season has gone.

Data from NRCS SNOTEL stations shows all basins in Montana, Wyoming, Colrado, and Utah between 98 and 135% of median SWE.  The fattest snowpack relative to median is in the upper elevations of the Lower Colorado-Lake Mean watershed, which gets most of its water from the mountains of far southwest Utah.  Other Utah watersheds are 100-117% of median.

Source: NRCS
Watersheds in California, Nevada, southern Oregon, southern Idaho, and New Mexico are generally at or below median, with the Salt and Upper Gila basins of Arizona running the farthest below average.  Note that the southern Sierra are sampled by the State of California and are not included in the NRCS analysis above.  

In the Wasatch range, SWE as a percentage of median varies from 70% at Ben Lomond Trail in the Ogden Valley to 121% at Snowbird.  To the east, most sites in the Uintas are at or above median, as are sites in the Oquirrh and Stansbury Mountains to the west.

Source: NRCS
Let's take a closer look at the two Wasatch extremes relative to median: Ben Lomond Trail and Snowbird.  Ben Lomond Trail is low in elevation (5829 ft) and thus the snowpack evolution there is strongly influenced by temperature and precipitation type (rain or snow) in addition to total accumulated precipitation.  In mid February, this season's SWE (blue) was very close to median (purple), but then it flatlined for 2-3 weeks before declining through mid March and recovering slightly in the last few days.

Here, the warmth of march took its toll, as it did elsewhere in the other mountain valleys of the Wasatch. 

In contrast, at higher elevation (and north aspect) Snowbird, although the rate of increase in SWE has been lower than it was through early February, there have been no loses and SWE currently sits very near the peak median for the season (which occurs around May 1st).  

Source CBRFC
My view is that it has been a solid but not exceptional snow season by long-term historical standards.  Positives are the fact that much of the snow fell during the part of the season when the sun angle is low (i.e., December and January) and that it was a Steenburgh winter, with 100" of base reached at Alta before February 10th.  Negatives are that the spigot closed a bit from mid February to mid March. 

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