Winter Weather Awareness Week
December 7-11, 2009
Information on Downslope Windstorms
Backcountry and Avalanche Safety :
Active avalanche rushing down the side of a mountain.
Nearly all avalanches that involve people are triggered by the victims themselves or a member of their party. Therefore, if you know how to recognize avalanche danger, you can avoid it. A word of caution: Decision making is not always straightforward. You should take an avalanche class and you should always travel with experienced people to avoid putting yourself in greater danger, or avoid not knowing what to do if faced with a backcountry crisis.
The facts about avalanches are simple: they are hazardous or deadly if encountered. In the 2002-2003 winter seasons, there were a total of 58 people killed by avalanches: 25 backcountry skiers, 23 snowmobilers, 5 climbers, 4 backcountry snowboarders, and one hiker/snowshoer. Avalanches involving people DO NOT happen by accident. Three variables interact to determine whether an avalanche is possible:
- Terrain – the slope must be steep enough to have an avalanche
- Snow pack – The snow must be unstable enough to have an avalanche
- Weather – Weather is extremely important. Changing weather can quickly increase instability.
How do avalanches occur?
Avalanches can only occur on slopes steeper than about 25 degrees, and they are most common on slopes between 35 and 45 degrees. This is because slopes less than 25 degrees are typically not steep enough to slide, and slopes greater than 45 or 50 degrees tend to sluff snow constantly, which prevent large slabs from avalanching. Determining factors of whether a slope can slide are the steepness of the slope, its aspect (which direction it faces), the slopes shape, and its natural features. Steepness is the most important factor to consider. Most skiers and snowboarders tend to recreate on slopes between 30-40 degrees, where avalanches are most prone.
Diagram of how avalanches occur.
Where can YOU find a backcountry forecast?
The National Weather Service in Reno issues a Backcountry Forecast specifically for the Sierra from approximately December 1 st through May 1st. The forecast focuses on weather elements used for avalanche forecasting by the Sierra and Central Sierra Avalanche Centers and also for those who venture out into the backcountry. The forecasts focus on high elevation winds and snow. The following link will take you directly to the Backcountry forecast page from the National Weather Service in Reno:
http://www.wrh.noaa.gov/rev/avalanche/
A few more helpful websites to learn more about where and why avalanches occur, or about general avalanche conditions this winter, visit the following websites:
http://www.esavalanche.org/
http://www.sierraavalanchecenter.org/
http://www.fsavalanche.com/
http://www.shastaavalanche.org
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www.weather.gov 
