Reading the Snowpack
Reading a snowpack can be as much art as it is science. We talk with Eric Trenbeath, avalanche forecaster for the USFS Utah Avalanche Center for the La Sal and Abajo Mountains, about reading patterns in the snow and how that fits into an avalanche forecast. Each snow or wind event change the story and ultimately change the daily forecast.
Meet the Scientist: Eric Trenbeath
Eric Trenbeath is an avalanche forecaster for the USFS Utah Avalanche Center for the La Sal Mountains near Moab. A Utah native, He grew up skiing in the Wasatch Mountains of northern Utah. In 1989 he landed his dream job as a live-in cook with a ski pass at the Goldminer’s Daughter in Alta, Utah which led to a 10 year career on the Alta Ski Patrol. In the summers he worked as a guide on the Colorado River. Calling Moab his year-round home since 1999, he forecasted in the La Sals from 1999-2003, and, after a several year hiatus, returned as the forecaster in 2013. He works as a ranger in Bears Ears National Monument in the off season.
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Interview Transcript - The Art of Avalanche Forecasting
ScienceMoab The whole basics of an avalanche is a big mass of snow, sliding down a hillside. So why would a mass start sliding?
Trenbeath What slides is a slab. What you need is a cohesive slab that’s a separate unit from the weak layer that’s underneath. So in October, we’re getting these weak layers forming, let’s say at the ground layer. And then it starts to snow again. And the new snow that sits on top forms its own cohesive layer that separates from that weak layer underneath. And depending on gravity, the shape of the terrain, there’s some tension on this overriding slab and it’s kind of hanging in a state of suspension waiting for a trigger. And that trigger can come through the addition of more snow creating a heavier load or a person or a snowmobile or something walking on to that slab and causing it to break and fail on the weak layer underneath.
ScienceMoab You go out during the snow season and physically dig what’s called snow pits. What is the process of digging a snow pit and what do you get out of it in terms of understanding what’s what’s going on?
Trenbeath We dig down to the ground throughout the season until we get four or five feet of snow. we’re looking for various weak layers and striations in the snowpack. Each layer in the snowpack has been created by a different weather event: whether it’s a high pressure period that forms the faceted snow, whether it’s a sun crust that forms on the surface, whether it’s wind deposition, or even wind erosion. All these things create the different layers in the snowpack. And some are harder and some are weaker. When you look in the snowpack and see hard layers overtop of weak layers, that’s potentially an unstable situation. That’s what we’re looking for.
ScienceMoab How does all this information fit into a forecast?
Trenbeath You go out in the field one day, gather information, dig snow pits, chart the weak layer, see what the wind loading has been doing, see how the snow has changed throughout the day based on temperature, wind, sun exposure, then collect that data. The next morning, you look at what’s changed overnight. For instance, overnight, the winds were southwest at 15 miles per hour; then about midnight, they picked up at 35 miles an hour, so more snow is being transported. Then it started snowing at four o’clock in the morning and two more inches. And so then I add all that into what I saw the day before, and put out the daily forecast. Then I go out in the field again that day and see if what I said was correct and then collect data for the next day’s forecast.
ScienceMoab How does the snowpack differ here in the La Sal and Abajo Mountains, considering it’s kind of a snow island in the desert?
Trenbeath One of the big factors is the wind. Being an isolated mountain range out here in the desert, we can really get wind blasted. And that’s why people look up from town and they see all the rocks showing on the south and west facing slopes. And they think there isn’t any snow up there. But it’s on the north side and you get on the north side, there’s 5-8 feet of snow or something back there, you know, so the winds really affect the snowpack here. And then also, it’s kind of hit or miss whether a storm will hit us or not. If it’s just off track just a little bit we can get completely missed. We definitely have a little bit more of just a wildcard factor by being out here in the middle of the desert.
ScienceMoab Do you have any advice for folks who might be interested in snow and Avalanche forecasting?
Trenbeath The study of snow is pretty fascinating. It’s often referred to as snow science, but I think it’s as much an art as it is a science. The definition of art is really methods through observation. So when you look at what’s going on out there, it’s just about looking at these building blocks and looking for hard layers over weak layers and learning to recognize when conditions are changing, and anyone can do it. I would just encourage everyone to study the forecast if they want to learn more about the snowpack and how it works. Take an avalanche education class or in the field snow training. We offer several throughout the winter.
