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Jan

15

2013

Through Thick and Thin

risk-continuum-re-wikstrom-mountain

Fat snowpacks tend to be safer. But a two-foot slab is still a two-foot slab. Dean Collins sends a test block into the pit.

How weather shapes the snowpack.

By Jeff Burke | Photograph by Re Wikstrom

As backcountry travelers, we’ve learned to expect particular character traits in the snowpack of a given region. Thick is strong, thin is weak, as the adage goes. Often the good and bad are associated with specific locations. It’s not uncommon to hear a Colorado skier say “I’m going to coastal BC this year to find some bomber conditions.” Translation: “The thin and rotten snowpack of the Colorado Rockies is notoriously slide prone and weak layers tend to persist there all season, whereas the northern coastal ranges typically offer wetter and more frequent storms. When combined with consistent temperatures, this results in a less slide prone snowpack.”

A reminder from your introductory avalanche class—there are three geographical avalanche climate zones in North America: maritime, intermountain, and continental, each with their own general characteristics (note emphasis on “general”). Maritime climates are steadily warmer and wetter over the course of the winter season. The weather produces snowpacks that are thick (more than 2-3 meters deep) and considered strong because the snow that falls is plentiful, dense, and bonds well. Thicker snowpacks insulate the cold air from the warm ground and have a smaller temperature gradient, which influences change in snow crystals. The result is usually fewer rotten layers. Conversely, continental climates are far from the ocean, and often have temperature extremes (cold nights thanks to radiation cooling, and warm days thanks to brilliant sunshine), long drought spells between storms, and thin snowpacks (less than 1.5 meters deep). So they’re weak and unstable by comparison. Intermountain snowpacks are often a mixed bag of the two. While it’s generally accepted that a thick snowpack is superior to a thin one, the more important consideration is the kind of weather that designs, builds, and influences a snowpack over time.

Just because you’re in Washington doesn’t always mean you’re skiing maritime snowpack, and just because you’re in Utah doesn’t mean you’re skiing continental snowpack. “If you have variability in the weather, with varied snowfall types, and big temperature swings, there will be large variability in the snowpack,” says meteorologist and mountain weather specialist Jim Woodmencey.

Last year, thin snowpacks and/or persistent weak layers prevailed in much of the West, from the southern San Juans to the Monashees. Paying close attention to the weather trends (instead of assuming that the snowpack is predetermined by its region) can give a pretty good indication of what kind of snowpack you’re facing over a given season.

We want to be decisive in our advice in these pages, but paying attention to the weather, following the avalanche center reports religiously, and remembering the thick versus thin adage isn’t enough. Because there’s a rub: “The caveat in all of this,” says Chris McCollister, an avalanche forecaster for the Bridger-Teton National Forest Avalanche Center, “is that you can have a thick, dense snowpack sitting on top of a thin, weak one. Chances might be very low that you trigger a slide, but if you do, the consequences can be extremely high.”

More avalanche safetyFrom the Winter 2013 issue. 

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