Storm conditions will create new problems for the weekend but don't forget about the tricky persistent slab problem. Large, destructive avalanches remain possible.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
The cold front passes over the interior regions on Friday and should be finished by Friday evening. The Purcells region can expect 10-15mm of precipitation with freezing levels of 1200-1400m. Alpine winds are expected to be strong from the SW but may switch to NW at the tail of the storm. On Saturday, light precipitation (1-3mm) is forecast with freezing levels around 800m and moderate SW alpine winds. The next frontal system is expected for Sunday and is forecast to bring another 10-15mm of precipitation.
Avalanche Summary
No new avalanches reported on Wednesday. Explosive control on Tuesday produced two size 2 slab avalanches in the deeper western part of the region. These avalanches released on the mid-Dec layer down 90-100cm on steep north aspect slopes. Some natural avalanche activity is expected on Friday in specific areas including fast sluffing from steep terrain and wind slabs in leeward alpine features. Small avalanches have the potential to step down and trigger a persistent slab which would greatly increase the consequences. Skier triggering is likely in wind loaded areas and steep terrain, and remains possible for persistent slabs where the tricky mid-December layer is still intact.
Snowpack Summary
The snowfall is burying widespread 10-20mm surface hoar, a sun crust on steep sun exposed slopes, and/or wind affected surfaces in the alpine. The problematic mid-December surface hoar/crust layer is typically down 40-80cm below the new snow and remains sensitive to human triggering is some areas. The reactivity of this persistent weak layer appears to be quite variable but still has the potential for large, destructive avalanches in some areas. The layer appears to be the most reactive at and below treeline. The mid-Nov weak layer is near the bottom of the snowpack but has generally become inactive.
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.
Loose Dry
Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.