Backcountry Advisory 

As of Wednesday, Dec. 20

Now that we have a winter snowpack, its important to think about how the new snow has affected the weaknesses that formed before last week. The storm that rolled through here last Friday packed a big punch. The warm moist air that fed the storm ran over the arctic air that had dominated our area for the previous week. This led to a strange temperature gradient from peak to valley.

Temperatures at 2,240 metres rose to -5 from -23 C in less than 24 hours. The temperature at 1,200 metres got to just below freezing while the valley was at about -10 C. The precipitation rate was between 3 and 4 cm per hour and winds in the alpine gusted to 147 km/h from the south.

Avalanche forecasters use a checklist where environmental variables are given positive or negative values with respect to their effect on the snow stability. All of the weather factors listed above are negative, and this load was added to an already weak snowpack.

The stability on Saturday was very poor, with widespread natural activity from below treeline up to the alpine. Much of this activity happened during the storm, allowing these slopes to reload. When the dust settled we saw limited activity with explosives testing in the ski areas, although "whumfs" and glide cracks persisted. Since then the storm snow has been settling rapidly, creating a somewhat strong layer over the pre-storm weaknesses.

There have been reports of skier-triggered slabs that may have run on the facets over the week, as well as occasional large, natural slab releases from rocky terrain. More stormy weather is forecast to roll into the area later this week, adding to the load.

The avalanche danger rating for the Whistler area on Wednesday, December 20 is MODERATE, with natural avalanches unlikely and human-triggered avalanches possible. There are numerous weaknesses that are still of concern at various elevations for different reasons. Check with the Canadian Avalanche Association public bulletin or the local ski patrol for more information.

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