Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Monday, November 23, 2009

Precision: Forecasting For Large, Medium, And Small Areas

Wish I knew what you were looking for, might have known what you would find—The Church

NOTE: Backcountry avalanche forecasting is composed of four interconnected elements: goal, people, awareness, and uncertainty. The size of the forecast area determines the precision of the forecast. It is more difficult to develop a forecast for a small area but the precision of the forecast is usually much higher than forecasts for large areas. Forecast precision is linked to awareness and uncertainty—maintaining awareness and managing uncertainty are central tools for helping people successfully align their beliefs about instability with reality ( which is the goal of backcountry avalanche forecasting ).

Backcountry avalanche forecasting is concerned with the specific slopes where high-stakes decisions require more precise information than in available in the avalanche bulletin. Forecast precision is important because of its relationship to awareness and uncertainty.

For most people, the bulletin is available at the scale of an entire mountain range, and does not contain information about specific valleys or slopes. Therefore, information about specific valleys and slopes must be obtained from direct observations made in the field. Understanding how to prioritise your observations based on the size of the forecast area can greatly increase your backcountry avalanche forecasting skill. At the scale of an individual slope,variables such as slope angle, slope shape, and the snowpack on the slope ( and your desire to ski the slope ) are the primary concerns.

You must acquire this information in the field because the chaotic interaction between weather and rough mountain terrain causes complex changes in the existing snowpack and complex deposition patterns for new snow. Managing the resulting uncertainty, while maintaining an accurate sense of awareness about instability for a specific place and time, is the heart of backcountry avalanche forecasting.

Information Types And Size Of Forecast Area
Avalanche forecasting uses three types of observations: weather, snowpack, and terrain. The size of the forecast area determines how observations are prioritised. For regional and local forecasting, which consider large geographic areas, weather observations are the primary source of information, followed by snowpack observations, and finally terrain observations.

Backcountry avalanche forecasting is concerned with specific valleys or slopes. Here, terrain observations are the primary source of information, followed by snowpack observations, and finally, weather observations. To develop an accurate backcountry avalanche forecast for a single valley or slope, you need information relevant to the valley or the slope on which you plan to travel.

Table 1.1. Different scales of terrain with observations used in forecasting. For regional and local forecasts, a few very general terrain observations are used. This includes aspect and elevation band. For valley and slope forecasts, very specific terrain observations are used, including aspect, elevation band, shape, angle, and slope configuration. The priority of observations is reversed as the size of the forecast area decreases.
Forecast AreaRangeSizeObservation Priority
Regional1000 square kilometresLargeWeather, snowpack, terrain.
Local100 square kilometresMediumWeather, snowpack, terrain.
Valley10 square kilometresSmallTerrain, snowpack, weather.
Slope2-3 square kilometresTinyTerrain, snowpack, weather.

NOTE: Any observation that reveals direct information about snowpack instability, such as whumphing or cracking at the snow surface, automatically receives priority over any other observations. Revise your forecast immediately if you observe any absolute indicators of instability, or if you obtain such information from snowpack tests.

Scenario: Backcountry Avalanche Forecasting In The Cascades
A strong winter storm forms as a large mass of warm, moist air is forced over the Cascade Mountains. During the storm, office-based avalanche forecasters use meteorological data, along with knowledge of how air masses interact with mountain topography, to produce a forecast that is relevant for the Cascade Mountains in general. The bulletin may include information such as aspects where you might expect to find wind-loaded snow deposits, or other information about the snowpack that has been directly observed in some places. However, much of the snowpack data in the bulletin is derived from the weather observations before and after the storm.

After the storm, the bulletin rates the current avalanche danger as high above 4000 feet and considerable below. Excited by the prospect of fresh tracks, you and your friends discuss a trip to Snoqualmie Pass to ski the Phantom Trees backcountry ski run. The bulletin contains a set of generalisations that are accurate for the Cascade Mountains but may not apply directly to Alpental Valley, and will not contain any specific information on conditions near the Phantom Trees backcountry ski run. What is the appropriate course of action?

Table 1.2. Different forecasting data order for areas of decreasing size in the Cascade Mountains. Backcountry avalanche forecasting is only concerned with producing forecasts for valleys and slopes, not entire mountain ranges. However, information from wide-area forecasts may be very useful during the backcountry avalanche forecasting process.
Forecast AreaSizeType Of InformationSource Of Information
The CascadesLargeWeather, snowpack, terrain.Public Avalanche Bulletin
Snoqualmie Pass RegionMediumWeather, snowpack, terrainPublic Avalanche Bulletin
Alpental ValleySmallTerrain, snowpack, weather.Local Observations
Phantom TreesTinyTerrain, snowpack, weather.Local Observations

Figure 1.1. Regional. The Cascade Mountains in Washington State. Sometimes wide-area weather patterns create snowpack features, such as crusts, facets, or surface hoar, found across large areas of the Cascades. So what's the catch? A lot of information is missing. Little is known about the specific properties of the snowpack over most of the Cascades, and little is known about the specific properties of weather interaction with terrain during a given snow storm. While you can develop an accurate forecast of instability in general by observing the weather, you simply cannot create a precise forecast of snowpack instability for all the valleys and backcountry ski runs in an entire mountain range from weather data alone. Precise information on the existing snowpack and current snow deposition patterns is extremely important for backcountry avalanche forecasting. The bulletin does not contain precise information about the structure of the existing snowpack on specific slopes, nor about new snow deposition patterns on specific slopes. Existing snow structure and new snow deposition patterns are critical variables for avalanche formation.

Figure 1.2. Local. The mountains around Snoqualmie Pass. Even though this area is much smaller than the entire Cascade Mountains, the public avalanche bulletin still does not apply to these groups of mountains except in a general sense. In this area, certain aspects and elevation bands may conform to the pattern of instability discussed in the bulletin. As you travel through the area, your observations will help you develop the first version of your backcountry avalanche forecast for the day. Do you notice large amounts of new snow? Recent avalanches of any type? Is it raining? Is it snowing? Is the ambient air temperature warm or cool? What is the expected air temperature trend? Weather observations are the priority since you are still dealing with avalanches on a large scale, but snowpack observations may be useful as well. Terrain observations are of little use since you won't be skiing in most of this terrain. ( Always follow the golden rule: pay very close attention to any observation, such as cracking in the snow surface or recent avalanches, that reveals direct information about snowpack instability. )

Figure 1.3. Valley. Alpental Valley. A backcountry avalanche forecast is usually prepared for a single valley, however some elements may be relevant to other valleys nearby. The public avalanche bulletin does not contain information about instability in this valley because office-based forecasters do not have precise information on the interaction between the weather and the terrain during the storm. Without precise information, office-based forecasters only have a rough, general idea about how the interactions between the weather and the terrain have affected the snowpack here. Combining information from the bulletin with your own observations helps refine your backcountry avalanche forecast even further: once inside the valley you can begin to examine the terrain and snowpack to see the actual conditions created by the recent weather.

Figure 1.4. Slope. Phantom Trees backcountry ski run. A slope forecast applies to a specific backcountry ski run inside the valley. The public avalanche bulletin does not contain information about instability for this ski run because office-based forecasters do not know exactly which slopes have been loaded by wind, nor do they know the exact locations of weaknesses and whether or not these weaknesses have the right structure, strength, and energy to release avalanches. Therefore, you must combine information from the public avalanche bulletin with your own observations to determine whether or not the snowpack is unstable inside this specific backcountry ski run. For example, a quick visual observation of the east bowl below the summit of Mt. Snoqualmie will provide information on whether or not the slope has been loaded ( is a cornice present? ) and whether or not there have been recent avalanches. This observation might tell you whether or not a lot of snow has been blown off the upper section of Phantom Trees, and gives you an idea about what you might find on the upper half of the mountain. Could there be a transition zone between the lower and upper section of the run where the snowpack is thin and weak? Several observations are noted in this image—it should be obvious that the bulletin does not contain information with this level of precision. By this point, you should have some degree of awareness about instability. However, it is still important to measure your residual uncertainty and make decisions accordingly.

With respect to signs of instability, absolutes are rare but they do exist.Any observation that reveals direct information about instability, such as whumphing or cracking at the snow surface, automatically receives priority over any other observations. Revise your backcountry avalanche forecast immediately if you observe any absolute indicators of instability, or if you obtain such information from snowpack tests.

Building A Nowcast From Your Observations
When it's time to ski up or down, you need a nowcast, which is your belief about patterns of instability on the slope you plan to ascend or descend right now. As you combine observations, whether from the bulletin or your own, consider whether or not the observations point toward instability. The distribution of weaknesses and the energy required to release avalanches changes with the passage of time and location, so you may need several nowcasts. For route selection purposes, it's useful to have a nowcast prior to uphill travel.

Figure 1.5. Distribution of weaknesses could range from very rare to very frequent. What do your observations suggest?

Figure 1.6. Triggering energy required to release avalanches could range from high triggering energy to low triggering energy. What do your observations suggests?

On the day of this ski tour, from observations collected prior to travel, it is reasonable to conclude:

  • Observations of the snow surface above treeline show significant snow drifts.
  • Relatively low energy triggering energy is required for avalanche formation in wind-deposited snow.
  • Stability is poor on steep slopes above tree line.
  • Because of the consistent temperatures during the storm, there are fewer weaknesses below treeline and slightly higher triggering energy is required for avalanche formation.
  • Stability is good on gentler slopes below tree line and fair on steeper slopes below treeline.
  • This only applies to areas where the snowpack is sheltered from the wind.
  • There is a transitional region where stability is fair to poor on moderate and steep slopes at treeline.
  • Ski quality is best below treeline.

The mix of stability ratings illustrates a key concept from this discussion: even at the smallest scale, one size rarely fits all.

Your perception of instability is likely to be at its worst when snow quality is very good but instability is relatively difficult to find or trigger. Understanding forecasting procedure for geographic areas of various sizes helps you maintain awareness and manage uncertainty by teaching you how to effectively prioritise your observations.

Research by Bruce Jamieson ( which may be relevant only in British Columbia ) shows that the public avalanche bulletin for the Columbia Mountains tends to predict higher regional danger levels than you might actually find in the field. However, local danger can be much higher on specific terrain features. This is why it is very important to combine information from the public avalanche bulletin, which covers a large geographic area and does not account for specific terrain features, with information that matches the much smaller size of a backcountry avalanche forecast region.

At which location is your perception of instability poorest? Below treeline or above treeline? Where are you most likely to make a mistake?

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