Avalanche Forecast
Purcells
Reactive new storm slabs have formed on an unstable snowpack that is capable of producing very large avalanches.
A Special Avalanche Warning is in effect in this region. Copy this address to view details: http://bit.ly/2nSOUyX
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- If triggered, storm slabs may step down to deeper layers and result in large avalanches.
- Choose shallow-angled and sheltered terrain where new snow hasn't been wind-affected.
- Avoid exposure to sun-exposed slopes.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Several unstable weak layers are lurking deep in our snowpack. These layers have the potential to be triggered both naturally and with skier or rider traffic. When triggered, these layers have been producing very large avalanches that can run far.
Travel and Terrain Advice
- Use conservative route selection. Choose low-angle, supported terrain with low consequence.
- Avoid the runout zones of avalanche paths. Avalanches could run full path.
Avalanche Summary
Wednesday's reports showed ski cutting producing storm and wind slab releases from size 1-1.5 as well as an older size 2.5 natural wind slab release. Wind slabs were focused on northeast aspects and storm slabs on south aspects. We have one report of an older size 3 deep persistent slab that released from a south aspect at 2600 metres.
Many small to large (size 1.0 to 2.5) slab avalanches were observed on Tuesday, including storm, wind and persistent slabs. The storm/wind slabs were 10 to 40 cm deep, on all aspects, all elevation bands, and triggered by skiers and explosives. The persistent slabs released on the early-January layer, were triggered by explosives, and were generally 100 to 120 cm deep. Similar avalanches were reported between Friday and Monday, showing a steady trend of activity.
Expect dangerous snowpack conditions to persist on Friday as our snowpack adjusts to the new load. A stable weather pattern in the forecast suggests we may see a decrease in natural avalanches, but human triggering remains a big concern. Storm and wind slabs also have the potential to step down to deeper weak layers, which could produce very large avalanches with high consequences.
Snowpack Summary
Roughly 20-30 cm of new snow fell during Wednesday's storm and buried preexisting storm and wind slabs. Collectively this storm snow overlies a complex and generally weak snowpack structure with four active weak layers that we are monitoring:
1) 90 to 140 cm of snow now overlies a crust and/or surface hoar layer from mid-January. The crust is reportedly widespread, except for possibly high elevation north aspects. The surface hoar exists at all elevation bands.
2) The early-January persistent weak layer is 100 to 160 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is found at all elevation bands.
3) Another weak layer buried mid-December consists of a facet/surface hoar/crust combination, which is buried 120 to 180 cm deep. It is most problematic at and below tree line.
4) A crust/facet layer from late November is yet another weak layer, which has recently produced very large, destructive avalanches.
The wide distribution and ongoing reactivity of these layers suggests that choosing simple terrain free of overhead hazard is the best avoidance strategy.
Weather Forecast
Friday: A mix of sun and cloud, gradually clearing overnight. Light north winds. Alpine high temperatures around -13.
Saturday: Mainly sunny with some valley cloud due to a building temperature inversion. Light northwest winds. Alpine high temperatures around -10, cooler at lower elevations.
Sunday: A mix of sun and cloud with thicker valley cloud due to a lingering temperature inversion. Light southwest winds. Alpine high temperatures around -10, slightly cooler at lower elevations. Temperature inversion breaking down overnight.
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Variable snowfall and steady winds have formed reactive new storm slabs on the surface. As the new snow consolidates, potential for human triggering storm slabs will increase. Strong solar effect may act as a trigger for natural avalanches on Friday.