Avalanche Forecast

High -

Wednesday night: Cloudy with continuing flurries bringing 5-10 cm of new snow. Moderate west winds, becoming strong at ridgetop.

Thursday: A mix of sun and cloud and 2-day snow totals of 20-50 cm. Flurries beginning again in the evening. Moderate to strong northwest winds. Alpine high temperatures around -14.

Friday: Cloudy with continuing flurries bringing about 5 cm of new snow, totaling up to 15 cm with overnight accumulations, easing again overnight. Light to moderate southwest winds, becoming strong at ridgetop. Alpine high temperatures around -3 with freezing levels rising to 1500 metres into the overnight period.

Saturday: Mainly cloudy with scattered flurries bringing 5-10 cm of new snow. Moderate west winds. Alpine high temperatures around -6.

Recent avalanche activity in the Purcells has been simultaneously impressive and terrifying. Explosive control work and other large triggers have consistently produced large persistent slab avalanches to size 3 (very large!) and larger on all aspects in alpine terrain. 

A couple of recent MIN reports describe a very large (size 3) deep persistent slab on a well-trafficked slope remote triggered by one or more snowmobiles in the Lang Creek area on Sunday. An adjacent second avalanche may have been triggered sympathetically by the first. On Monday another large (size 2) deep persistent slab was observed just below Gorman Lake. Its trigger is unknown.

Common characteristics of recent avalanches in the region include wide propagation, remote triggers and full depth avalanches scouring away the snowpack to ground.

Looking forward, concern for very large deep persistent slab releases has been increasing in line with new snow accumulation, which is forecast to continue through Wednesday night. Not only does the new snow represent a problematic new load on our weak snowpack, but smaller storm slab or wind slab releases may begin to act as a trigger for very large avalanches.

A variable 10-35 cm of new snow has buried surfaces over the region that mainly consist of widespread surface hoar ranging from 5-20mm in size. Sun crusts were also observed on steep south aspects in advance of the snowfall.

The Purcells received 60-120 cm of snow from the big storm two weekends ago. This snow has since settled into a slab over a couple of buried surface hoar layers 70-180 cm below the surface. These layers are expected to be progressively gaining strength, but the character of the lower snowpack complicates matters.

The base of the snowpack in the Purcells is astonishingly weak, much weaker than in an average season. This weakness is widespread across aspects and elevation bands, meaning it's almost everywhere. This basal layer consists of crust, facets and depth hoar. 

With the addition of the new snow last weekend, this weakness became overloaded and its failure has resulted in some spectacularly large and destructive avalanche activity. 

• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
• Avoid being on or under sun exposed slopes.
• Remote triggering is a concern, watch out for adjacent and overhead slopes.
• If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.