Avalanche Forecast
Purcells
Very dangerous snowpack conditions are widespread. Deeply buried weak layers are at a tipping point and producing historic 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
- Avoid exposure to sun-exposed slopes.
- Choose shallow-angled and sheltered terrain where new snow hasn't been wind-affected.
- If triggered, storm slabs may step down to deeper layers and result in large avalanches.
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
- Avoid the runout zones of avalanche paths. Very large avalanches have been running full path.
- Use conservative route selection. Choose low-angle, supported terrain with low consequence.
Avalanche Summary
We have preliminary information of a snowmobiler triggering a size 2 avalanche on a north aspect at 2200 metres in the Hospital Creek area east of Golden on Friday. The individual was potentially deeply buried by the debris and was extricated by their riding companions. The storm slab had a crown depth of 40-60 cm and width of 50 metres. Although this incident did not occur in a forecast region, it gives an indication of potential conditions in the surrounding area.
Reports from Thursday included observations of numerous natural and explosives-triggered storm slab releases, generally from size 1.5-2.5. Several more explosives-triggered persistent slabs ran from size 2-3 with crown fracture depths of up to 150 cm and runout distances of up to 400 metres. The adjacent Columbias forecast regions saw storm slab and persistent slab avalanches reaching size 4 and greater. Several historic slides destroyed mature timber along their edges as well as on opposing slopes.
Looking forward, it will be essential to maintain elevated caution as ratings shift into the CONSIDERABLE range. As the likelihood of triggering diminishes, the destructive potential of avalanches will remain dangerously high.
Snowpack Summary
Recent accumulations of 20-30 cm have formed reactive new storm slabs on the surface throughout the region, burying pre-existing 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
Saturday: Mainly sunny with some valley cloud due to a weak temperature inversion. Light northwest winds, moderate to strong at high elevations. Alpine high temperatures of -11, slightly cooler at lower elevations.
Sunday: A mix of sun and cloud with cloud increasing over the day. Light variable winds. Alpine high temperatures of -12. Cooling over the day as the temperature inversion breaks down.
Monday: Mainly sunny with some valley cloud due to a new temperature inversion establishing. Light north winds, increasing overnight. Alpine high temperatures of -10, cooler at lower elevations.
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Heavy snowfall and steady winds have formed reactive new storm slabs. Natural avalanches have been numerous and the potential for human triggering is high. Strong solar effect and alpine winds may act as a trigger for natural avalanches on Saturday.