Watch for wind slabs in unusual places as we head into a clear cold week. Be extra cautious in thin snowpack areas where it may be possible to trigger deeper weak layers.
Confidence
Moderate - Wind speed and direction is uncertain
Weather Forecast
MONDAY: Mix of sun and cloud, light east winds, alpine temperatures around -20.TUESDAY: Sunny, light northeast winds, alpine temperatures around -18.WEDNESDAY: Mix of sun and cloud, 20-40 km/h north winds, alpine temperatures around -16.
Avalanche Summary
On Saturday, explosive avalanche control produced several size 1-2 slab avalanches on wind loaded features and some larger natural avalanches were observed on steep south facing slopes. MIN reports from the past few days suggest the recent snow is still reactive to rider triggers, including a report from the Kicking Horse area of a skier caught in a small slide on a southwest aspect and another report from from west of Kimberly where a ski cut in a cutblock resulted in a size 2 persistent slab avalanche.Looking ahead to Monday, expect wind slabs to remain reactive to human triggers. The additional load may also make the persistent slab over the mid-December interface more reactive in thin snowpack areas.
Snowpack Summary
The region received 40-60 cm of new snow between Christmas and New Years. This snow has been redistributed by strong winds that have come from a variety of directions and loaded leeward and cross-loaded features. Sunny skies will promote settlement of the snow on open south-facing slopes. The mid-December facet/surface hoar interface can be found buried 40-80 cm deep. This layer has shown signs of being poorly bonded to the overlying slab in snowpack tests, and has produced intermittent avalanche activity in shallow snowpack areas. The lower snowpack is well bonded and features a thick rain crust near the ground that appears to be dormant.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.