Avalanche Forecast

Moderate - Forecast snowfall amounts are uncertain.

Thursday night: Becoming cloudy with increasing flurries bringing 5-15 cm of new snow. Light to moderate southwest winds, becoming strong at ridgetop.

Friday: Cloudy with continuing snowfall bringing 10-25 cm of new snow, transitioning to rain below about 1900 metres. Strong southwest winds. Alpine high temperatures reaching 0 to +1 as freezing levels climb to 2000- to 2300 metres over the day.

Saturday: Continuing snowfall bringing 10-20 cm of new snow to the alpine, rain below about 1700-1800 metres. Moderate souhwest winds, becoming strong at ridgetop. Alpine temperatures around 0 with freezing levels beginning to fall from 2000 metres in the afternoon.

Sunday: Mainly sunny. Light southwest winds. Alpine high temperatures around -9.

On Sunday there were three deep persistent slab avalanches reported in the region. Two were triggered by explosives, but one that occurred near Golden was remotely triggered by humans. This activity outlines the fact that this layer remains a problem in the region. Large alpine features, especially rocky, thin snowpack areas are the most likely places to trigger this layer.

Over the past week there have been a few reports everyday of natural, human and explosives triggered storm slab and wind slab avalanches up to size 3, but mostly around size 1-2. These have occurred on all aspects, mainly at treeline and alpine elevations. The peak of this activity occurred on Saturday and has been tapering since. Wind loaded areas at treeline and in the alpine remain the most likely places to trigger an avalanche.

Looking forward, heavy precipitation, strong winds, and rising temperatures will promote increasing natural avalanche activity on Friday. During this time, deep persistent slab releases such as those mentioned above may result from avalanche activity in surface snow layers.

A variable 15-40 cm of new snow is expected to fall at the highest elevations of the region by end of day on Friday. The new snow will cover wind affected recent snow at alpine and upper treeline while rain saturates the surface below about 1900 metres.

A weak layer of surface hoar can currently be found about 70 cm deep around Golden and Kimberley, 30 cm deep around Invermere, and 100 cm deep along Kootenay Lake.

As usual for the Purcells, the base of the snowpack contains basal facets and it remains possible to trigger these deep weak layers in shallow rocky start zones or from a heavy trigger such as a cornice fall or explosives.

New snow, rain, and warming will collectively add considerable strain to these weak layers. Their potential to fail naturally and produce large, destructive avalanches will be significantly increased during the storm.

• Storm slab size and sensitivity to triggering will likely increase through the day.
• Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
• A moist or wet snow surface, pinwheeling and natural avalanches are all indicators of a weakening snowpack.