Overview

Avalanche Bulletin User Guide

For the public avalanche bulletin for your region, call 1.800.667.1105, or view them online on the Latest Bulletins page.

Using an avalanche bulletin is a skilled activity. Surveys show that it takes training to understand even the basic meanings of the danger ratings contained in the bulletin. Here is an introduction to some of the concepts that are fundamental to Canadian avalanche forecasts, bulletins, advisories and reports.

Avalanche bulletins, also known as avalanche advisories and avalanche forecasts, include danger ratings as well as discussions prepared by professional forecasters. Formats vary but they usually provide clear information on what the primary concerns are for the day, travel advice, information on avalanche activity and snowpack conditions, as well as forecast weather conditions. Avalanche danger ratings use a five-point, internationally recognized scale that includes a danger level, travel advice and a description of the expected likelihood, size and distribution of avalanches.

In Canada, avalanche danger ratings are generally provided for three elevation bands:

Below treeline example

Below Treeline ratings are applicable to densely forested areas that may have openings and glades within them large enough to create relatively small avalanches. Avalanche runout zones from higher regions may run through this elevation band.

 

Treeline example

Treeline ratings are applicable to the transition area between densely forested areas below and alpine regions with few or no trees above, also known as sub-alpine. Trees are generally smaller and are found in non-contiguous stands separated by open, wind-exposed areas. The treeline zone is a relatively narrow band compared to below treeline and alpine.

Alpine example

Alpine ratings are applicable to areas above the treeline. Small, isolated trees may be found at the low end of this elevation band but they quickly give way to large expanses of open slopes leading to ridges and peaks. This entire band is highly exposed to the effects of wind.

Avalanche bulletin production and limitations
For some bulletin regions, forecasters regularly go into the field to gather data. In others, forecasters spend some time in the field, but rely primarily on data supplied by others. In both cases, forecasters augment personal observations with information from other sources. These sources include daily industry information exchanges, manual or remote automated weather stations, weather maps, satellite images and public and professional reports. Forecasters then analyze, evaluate, and synthesize the raw data and local analyses, combine that with their personal experience, and generate an assessment of current snowpack structure and avalanche conditions for a region.

Next, weather forecast information is investigated and the avalanche forecaster writes an avalanche forecast. Sources of weather information include the public and private forecasts and analyses of weather forecast computer models.

After the avalanche forecast is completed, forecasters choose a rating from the danger scale that best describes the situation on a given day. There is no simple or clear-cut formula as the knowledge, judgement, and intuition of the forecaster are the primary tools used in this process.

Forecasters at work

The snowpack constantly changes over time. This process is driven by the weather and is influenced by internal snowpack conditions such as temperature, density and depth. Because the snowpack is constantly changing, avalanche danger also varies over time. This variability can occur very slowly where notable differences take weeks to evolve, or very quickly, where notable differences develop in only hours or minutes. Bulletins in general are less accurate when extrapolating into the future (for example, danger ratings on the day of the forecast are usually more accurate than danger ratings forecast for the next 24 or 48 hours).

The snowpack also changes from one place to another. These variations are caused by differences in the terrain such as elevation, aspect, terrain configuration, vegetation and incline. Because the snowpack varies over space, avalanche danger also varies across the landscape. Spatial variation sometimes occurs only over large distances where there is no significant change over hundreds or even thousands of metres, but can often be found over relatively short distances (tens of metres or less).

Bulletins are only as good as the data that goes into them. Many bulletins will include an indication of the forecaster's confidence in his or her predictions. Recreational users need to factor these limitations into their travel plans. Canada is a vast and untracked country, our bulletin regions are the size of entire European countries, and the avalanche safety data is sometimes very scarce. This means that backcountry users here need to share the responsibility to carefully assess whether it is a good time to attempt a given slope or line. The forecaster will let you know this situation, but it is up to you to factor in the appropriate margin for safety in your activity.

 

related links