Avalanche Forecast

Archived

Dec 7th, 2012–Dec 8th, 2012

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Confidence

Fair - Due to variable snowpack conditions

Weather Forecast

Overnight and Saturday: Light to locally moderate snowfall on Friday night with clearing on Saturday / moderate west winds becoming light and northwest with clearing / Freezing level at surfaceSunday: Light snowfall / Light northwest winds / Freezing level at surfaceMonday: Light snowfall / moderate west winds / Freezing level at surface

Avalanche Summary

Observations in the region have been limited; however, on Thursday explosives control produced avalanches to size 1.5 that ran within recent storm layers. There was also an isolated size 1.5 release that failed to ground in steep, unskiable, rocky terrain with crown depths to 85cm.Close to Invermere, a natural cornice release triggered a size 2 avalanche on a northwest facing alpine slope. The avalanche was about 120cm deep and ran on the early November faceted crust.

Snowpack Summary

In the upper snowpack light amounts of low density snow override wind slabs which were formed earlier in the week. In some areas, recent winds have also shaped unseasonably large cornices.Up to 75cm of snow that fell over the past week may sit above a surface hoar layer that was buried at the end of November. Although not widespread in the region, releases on this layer may be likely if you have it in your area. Another surface hoar layer that developed during mid-november may exist up to 100cm down, and is still on the radar of professionals on the east side of the region.Near the ground is the early November crust. There are some facets below it, or it may appear as a sandwich of crusts and facets. In some areas it may only now be primed for triggering with recent heavy loading. Observations have been limited; however, deep and destructive slides have been reported on this layer in a neighboring region.In general, significant variations in snowpack structure exist across the region. Digging down and making your own observations will be critical to safe slope selection.

Problems

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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.