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Avalanche Forecast

Archived

Jan 24th, 2015–Jan 25th, 2015

Alpine
Natural and human triggered avalanches likely.
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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

High freezing levels with rain and strong winds mean that the snow pack may take time to stabilize. Conservative decision making is still recommended.

Confidence

Fair - Freezing levels are uncertain

Weather Forecast

A second warm wet weather system will hit the coast on Sunday although a week ridge of high pressure will protect the interior from the brunt of this system and I expect the Purcells will see only light flurries or showers on Sunday. Warm temperatures and moderate to strong SW winds will persist through until Tuesday

Avalanche Summary

Parts of this region that see good amounts of snow/rain with warm temperatures are likely to see an active avalanche cycle. Areas that stay dry may not see natural avalanches, but human-triggered avalanches will still be possible. Avalanches ran in the region during and after the last major storm, typically running on the mid-January surface hoar layer.

Snowpack Summary

Between 10 and 20 cm of new snow fell in the last storm which I suspect is rapidly settling into a soft slab. This new snow sits above a moderately dense upper snowpack . A weak layer of surface hoar crystals lies buried between 35 and 50 cm below the surface. This "mid-January" weak layer has been responsible for most of the recent avalanche activity. While reports do indicate that this layer is starting to gain some strength and is becoming more difficult to trigger when ski-cutting small slopes, I suspect that we will see it produce avalanches through the weekend as the snow pack adjusts to the recent snowfall and continued warm temperatures. The mid-December surface hoar/crust layer is typically down 60-120cm below the surface. The reactivity of this deeper weak layer appears to be quite variable but snow pack tests indicate that it still has the potential to produce large avalanches in some areas. The layer appears to be the most reactive at and below treeline. The mid-Nov weak layer comprising facets can still be found near the bottom of the snowpack but has generally become inactive.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.