Bill Jennings: Snowpack means different things to different people

Just past midpoint in the season, we’ve already had a winter’s worth of snow in Spokane. At our local mountain resorts, the skiing and riding has been as good as it gets. But I was surprised to hear that the region’s snowpack is below average.

The fact that more mountain snow usually falls by now in any given year is an interesting paradox. For skiers and riders, after coverage builds to the point where rocks are buried and glades are open, the snowpack is as good as the latest storm.

However, drilling into a few details helped me understand that the snowpack has different meanings for different people. For people like managers of public utilities and farmers, it’s a critical guideline for spring runoff that will affect decision making during the next six months.

I scanned data online from local SNOTEL (SNOwpack TELemetry) sites. SNOTEL is a service provided by the Department of Agriculture’s National Resources Conservation Service (NRCS). More than 700 SNOTEL sites are installed in mountain watersheds across the country.

These automated outposts report the state of the snowpack in real time to the NRCS in kind of a sci-fi way. The data is sent via radio waves beamed off an ionization layer 50 miles high in the atmosphere, formed by billions of vaporized micrometeorite particles.

The snow station near Mount Spokane on Quartz Peak showed a depth of 42 inches on Feb. 1 – 76 percent of the average snowpack of about 55 inches. The average is based on winters from 1981 to 2010. Kellogg Peak’s depth of 31 inches was 47 percent of average. Schweitzer Basin showed 59 inches – 60 percent of average. The Lookout Pass SNOTEL showed 43 inches – 63 percent of average.

Given the epic skiing and riding conditions of late, these readings were puzzling. Perhaps an examination of how SNOTEL snow depth is determined could help explain what’s going on with the snowpack.

Snow depth is calculated by dividing the Snow Water Equivalent (SWE) by snow density. Snow density is an estimate of how much snow is actually water. In lieu of actual core samples, around here snow density is expected to be from 10 to 20 percent in the winter and 20 to 40 percent in the spring. SWE is an estimate of the total amount of water held within the snowpack.

For example, a snowfall of 25 inches at 15 percent density could be equivalent to 3.75 inches of rain (or runoff). Therefore, 3.75 divided by 0.15 is a snow depth of 25 inches.

The math seems superfluous – under normal conditions. It’s possible for the champagne powder we crave to spike the reported snow depth in a dramatic departure from reality. At 5 p.m. during last Sunday’s storm, the Quartz Peak SNOTEL reported an SWE of 12.7, 25 percent snow density: equal to a snow depth of 51 inches. At 6 p.m. the SWE increased to 12.8 and the snow density plunged to 8 percent.

Plug in the numbers and snow depth more than tripled, to 163 inches. Only an additional tenth of an inch of precipitation was recorded. In the real world, Mount Spokane reported a “paltry” 9 inches of fresh snow on Monday morning.

Math being math, the system will no doubt correct itself over time. Meantime, skiers and riders have been lucky to escape any significant rain events so far this season. But we may be in for some sloppy weather in the next few weeks. The longer view looks pretty good for the conditions, and for the snowpack to approach normal.

According to the NOAA Climate Prediction Center, a weak La Nina persists. A transition to neutral in the El Nino Oscillation Cycle (ENSO) is expected to begin in February, but it may not be enough to affect the pattern we’ve been enjoying. Elevated odds for below normal temperatures and above normal precipitation are predicted to continue for the Pacific Northwest through March and into April.