Summary:

Large natural avalanche (likely about size 3) on NW through W aspect in the Able Bowl in the Mount Cain area on Friday 18 March, 2011. No humans involved.

Details:

A large avalanche of note occurred in the Mount Cain area on Friday 18 March with details having come in only Tuesday 22 March, 2011. The slide took place some time in the afternoon on that day with an estimated 700-900m wide fracture line that spanned from NW through to W aspects in the Able Bowl below the true and false summits of Mount Able. The fracture line was about 200m below ridge line and was estimated to be between one metre and four metres in height. The avalanche ran the full path to fill the bottom of the bowl with deep debris and almost spilled over into the main east bowl drainage below the east bowl. One local described it as the largest avalanche he had seen in the Cain area in twenty years of skiing the area. The cornice above the slope was said by a number of observers to remain in tact meaning a cornice trigger was not the cause. Wind loading which had been happening for many days previously and continued on Friday is the probable trigger. The failure may have been wind loaded snow on the previous surface, or the failure may have been on our February crust layer deeper down. Further investigation is required.

This avalanche bulletin had the hazard in the alpine at CONSIDERABLE on that day. The part of the definition of this hazard rating under the "Likelihood of Avalanches" heading is that "natural avalanches are possible and human triggered avalanches are likely". This incident is a very good illustration of why it is so important to also make careful note of the "Avalanche Size and Distribution" descriptors in the Avalanche Danger Scale.  At a CONSIDERABLE rating this definition reads "small avalanches in many areas or large avalanches in specific areas or very large avalanches in isolated areas". Clearly there was a very large avalanche in an isolated area on this day. I mention this because it is important for users of the bulletin to recognise that even when hazard is dropping because natural avalanches are less likely and larger triggers may be required to start avalanches, large avalanches are still possible. It is important to read and understand all of the aspects of the hazard definitions (Likelihood of avalanches as well as size and distribution) and to read all of the details in the text of the avalanche bulletin and to apply them correctly to the terrain. We must not become complacent as hazard ratings drop.

It is also interesting to note that this forecaster departed from the Mount Cain area on that day likely within hours of the event, with a feeling of trepidation for what might happen at the mountain that week end. The wind had been blowing hard for most of the previous five days and slabs were fat and well primed for triggering while the wind continued to load. Additionally the wind was forecast to drop with a possibility for improvement in the weather and a big group of spring break skiers was arriving. This is the stuff that we forecasters can lose sleep over!

Reported by VIACS forecaster Jan Neuspiel with information from Cain local Cyril Douglas and observer Jim Stepan. Photos and further details to follow.