Avalanche Dangers in the Backcountry of the West
By Michael Ottenweller, Meteorologist, Medford NWS Office
As we move back into our winter season here in Southern Oregon and Northern California, we shift our focus to winter weather and especially snow. While the Rogue Valley generally does not see significant accumulations, plenty of locations throughout the area can see several feet during the season. Crater Lake, Mt. Shasta, and the Kalmiopsis Wilderness are a few spots that see abundant snowfall from now until the late spring. Many people enjoy venturing out to explore the snow on skis, sled, snowmobiles, or snowshoes; however, one often forgotten hazard of the back country this time of year is avalanches. On average, 25 people die each year in the United States from avalanches. A little education can go a long way in keeping you safe as you enjoy the wilderness this winter.
There are many components that lead to an avalanche. Terrain and slope angle are two critical factors that remain constant. The most dangerous slopes for producing avalanches are be-tween 35 and 50 degrees. Any-thing steeper than 50 degrees and the snow cannot hold to the face. Another important factor that does change is the type of snow. Unstable layers located deep beneath the surface are often the cause of larger ava-lanches. This can mean that even though snow has been on a mountain all winter long, it could still be unstable and dangerous. The last component to set the stage for an avalanche is the weather. Has the wind been blowing? Has it been snowing heavily or raining? Has the tem-perature changed drastically in a short period? All of these can be clues to increased potential for avalanches. Wind can move massive amounts of snow from one slope to another. Heavy snow or even rain on top of one of those weak layers can produce a prime avalanche environment, while rapid temperature changes can also make the snowpack more unstable.
Even though winter has not brought much snow to the area so far this year, that could all change very quickly. Crater Lake National Park had 25 inches on the ground in early December, which is 71% of normal. Mt. Shasta Ski Park has 10 inches, and Mt Ashland Ski Area has a base between 24 and 33 inches. With a moderate La Niña pre-sent, the forecast remains for a colder-than-normal and slightly wetter-than-normal winter, meaning even though not much snow has fallen yet, any snow that does fall is more likely to stick around for the winter months. If last season's La Niña is any indicator, where Crater Lake received a whopping 672.5" of snow, we should still expect some wet weather. This early December cold and dry pattern can make ideal condi-tions for hoar frost. Hoar frost can produce a perfectly unstable layer that will be ripe for release when loaded with heavy snow.
90% of avalanches with loss of life are caused by humans. What can you do to stay safe when traveling in the back country this winter? Stay alert to changing snow stability and recent weather conditions. Travel in a group, but only approach potential ava-lanche slopes one at a time. Look for the tell-tale signs such as recent avalanches, cracks in the snowpack, and wind-blown snow cornices. Always carry the proper equipment, including an avalanche beacon, shovel, and probe. Know how to use each tool and practice before you go. If you are caught in an ava-lanche, perform a swimming motion and try to reach a portion of your body sky-ward just as the snow is settling.
Education is your best defense against avalanche danger. There are several websites with current conditions and more information: http://www.avalanche.org/, http://www.shastaavalanche.org/advisories/advisories/avalanche-advisory, and http://avalanche.state.co.us/index.php are just a few. These websites also have information on when classes will be held near Mt Shasta. These classes are the best way to further your knowledge and practice your rescue skills. Enjoy the back country and stay safe this winter.
Source: http://www.wrh.noaa.gov/mfr/skywarn/winter2011.pdf
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