Avalanche Forecast
Purcells
Touchy wind slabs sit above several deeply buried weak layers that are reactive to human triggers. Be disciplined: perform cautious route-finding and limit overhead exposure, particularly on solar slopes receiving direct sun.
Avalanche Problem 1: Wind Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Seek out sheltered terrain where new snow hasn't been wind-affected.
- If triggered, wind slabs may step down to deeper layers and result in very large avalanches.
- Use ridges or ribs to avoid pockets of wind loaded snow.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Several weak layers are lurking in our snowpack, which continue to produce very large avalanches with high consequences. Limit your exposure to avalanche terrain and avoid overhead hazards.
Travel and Terrain Advice
- Make conservative terrain choices and watch for clues of instability.
- Avoid open slopes and convex rolls where buried surface hoar may be preserved.
- Be very cautious below treeline in open areas where surface hoar may be preserved.
Avalanche Summary
Numerous small to large avalanches were reported on Sunday, including storm slabs, wind slabs, persistent slabs, and deep persistent slabs. The largest avalanche was a size 4 natural avalanche on a northwest aspect at 2500 m, 200 cm deep, running for 1700 m, releasing on the late-November crust/facet layer described in the section below. The remainder of the reported avalanches were size 1 to 2.5 on all aspects, 1900 to 2600 m, 20 to 100 cm deep, which were triggered naturally, by skiers, and large loads. Cornices were observed to release naturally and by explosives, which triggered wind slabs below. Similar avalanches were reported on Saturday, showing a steady trend of avalanche activity.
Looking forward, dangerous snowpack conditions will persist in the region until a more stable weather pattern governs and we see a decrease in avalanche observations. All of our buried weak layers (described below) continue to produce large, destructive avalanches from natural and human triggers. Recent storm slabs and wind slabs have the potential to step down to deeper weak layers, which could produce large, destructive avalanches.
Snowpack Summary
Recent storms have produced variable new snow accumulations of 20-50 cm. Strong winds have produced wind slabs in lee features. This snow overlies a complex and generally weak snowpack structure with four active weak layers that we are monitoring:
1) 70 to 110 cm of snow 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 is 10 to 30 mm in size and exists at all elevation bands.
2) The early-January persistent weak layer is 80 to 130 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. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential.
3) Another weak layer buried mid-December consists of a facet/surface hoar/crust combination, which is buried 100 to 150 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.
Weather Forecast
TUESDAY: Partly cloudy, light to moderate northwesterly winds, alpine temperature near -10 C, freezing level below valley bottom.
WEDNESDAY: Cloudy with snowfall, accumulation 2 to 5 cm, moderate westerly winds, alpine temperature near -5 C, freezing level near 1400 m.
THURSDAY: Cloudy with snowfall, accumulation 5 to 15 cm, moderate westerly winds, alpine temperature near -10 C, freezing level near 1400 m and dropping to below valley bottom.
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Recent snow and strong southwest winds have built touchy wind slabs in lee features. Slabs exist on all aspects in areas that received more snow from the recent storms. Be particularly cautious on slopes that receive direct sunshine on Tuesday.