BY RANDALL OSTERHUBER
Old man winter packed a punch last December with early season storms that made for a truly white Christmas and sent powder hounds to the slopes in droves. Then, just as suddenly as he blew into town, he departed, leaving in his wake a string of sub-zero nights. Skating on Donner Lake was a treat for many, but the season has so far offered little more in the way of big dumps that skiers and the resorts were hoping for. The daffodils started to sprout this week, leaving many to wonder: What happened to winter?
For some perspective, Moonshine Ink turned to Randall Osterhuber, the snow guru up on Donner Summit who monitors the Sierra snowpack at UC Berkeley’s Central Sierra Snow Laboratory. In a ski town, he points out, not all precipitation is created equal:
The vast majority of the snows we’re skiing today fell during just a handful of days way back in December. Whether any winter has one or three — or eight — of these large snowfall events is what separates a powerful snowfall year from a modest one.
Each winter’s weather also has a highly subjective quality: how we, as snowplow drivers, skiers, or reservoir managers, interact with the snow. Large April snowstorms are of much less value to the ski area manager than the reservoir manager. This would not necessarily be revealed by the snowfall totals for any given season.
One reason we don’t have those cute Utah “Greatest Snow on Earth” license plates is that it rains mid-winter in the Sierra. Again, this type of precipitation provokes very different responses whether you’re a skier, an avalanche forecaster, or a trucker traversing I-80. So a winter is not only defined by how much precipitation falls, but by its timing and its type.
How is Winter 2013 stacking up against the mathematically average year? Well, every winter is different, and this one is no exception. Right off the bat, a huge variation in snow cover across the Tahoe Sierra was introduced by the early December warm storms. Big snowpacks up high, rain and virtually no snow down low. Today, this is still reflected in snow depths twice as deep above 7,500 feet in elevation than below that level. As of this writing (March 18), the data collected at the main study site of the Central Sierra Snow Laboratory (elev. 6,900 feet) reveals that cumulative precipitation is 86 percent of average; cumulative snowfall is 61 percent of average; and snow depth is 49 percent of average.
As a consequence of the railroad punching through, there has been year-round human presence on Donner Summit since the mid 1860s. Thoughtful railroad workers started collecting information about Donner Summit’s annual precipitation starting 1871, and snowfall data in 1878. This is an unusually long data set and is unique in the mountainous West.
If we compile and combine all the winter data from those early years until now, we can start to draw a picture of what an “average” winter looks like on Donner Summit. That is the beauty of mathematics. The horror of mathematics is that not a single one of those 135 winters will look like the mythical “average” year.
What the weather data does give us is a “mathematical average,” something very different from what is measured on the ground. If, for example, we look at each individual winter day’s snowfall over the course of the record — all the December 30ths, all the February 2nds, all the March 17ths, etc. — the snowfall data would paint an “average” picture of a little bit of snow falling every day up until about April 1, then the resulting snowpack melting, a little at first, then more rapidly as warmer weather gains traction. But during no winter will it snow a little every day. Our winters are made up of fairly large, discrete snowstorms.
I have no idea how the rest of the snow season will play out, but what I do know is that storm or sun, warm or cold, it will be anything but average.
