Avalanche Forecast
South Columbia
Touchy storm slabs sit above several buried weak layers that are reactive to human triggers. The danger will be HIGH if clear skies prevail, with direct sun on solar slopes. Be disciplined: perform cautious route-finding and limit overhead exposure.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Choose sheltered terrain where new snow hasn't been wind-affected.
- If triggered, storm/wind slabs may step down to deeper layers and result in very large avalanches.
- Use conservative route selection, choose moderate-angled and supported terrain with low consequence.
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 that are propagating far, 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
On Sunday, numerous slab avalanches were reported in the region. Storm and wind slabs were small to large (size 1.0 to 2.5) at all elevations and aspects and triggered naturally, by skiers, and explosives. Persistent slab avalanches were reported on the weak layers described in the section below, 100-200 cm deep. They were generally large (up to size 3.5), on north to easterly aspects, between 1400 and 2300 m, and triggered naturally and by large loads. A good example of skier-triggered storm slabs that stepped down to a deeper weak layer is described here. 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
Around 100-200 cm of snow from the past two weeks has formed a storm slab and wind slabs in lee features, which sit over an unstable snowpack. There are four active weak layers that we are monitoring:
1) 80 to 150 cm of storm snow sits on the 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 5 to 20 mm in size and was reported up to tree line elevations and possibly higher. This layer is the primary weak layer of concern in the South Columbia’s and remains within the depth range of human triggering.
2) The early-January persistent weak layer is 120 to 170 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 150 to 200 cm below the surface. It is most problematic at and below tree line.
4) A crust/facet layer from late November is yet another failure plane responsible for recent very large avalanches.
Weather Forecast
TUESDAY: Partly cloudy, light to moderate northwesterly winds, alpine temperature near -9 C, freezing level below valley bottom.
WEDNESDAY: Cloudy with snowfall, accumulation 5 to 15 cm, moderate westerly winds, alpine temperature near -2 C, freezing level near 1500 m.
THURSDAY: Cloudy with snowfall early morning, accumulation 10 to 25 cm, moderate to strong westerly winds, alpine temperature near -6 C, freezing level near 1500 m.
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Recent snowfall has formed storm slabs that will likely remain reactive to human triggering. The snow fell with strong winds, producing touchy wind slabs in lee features. Be particularly cautious on slopes that receive direct sunshine on Tuesday.