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Avalanche Forecast

Archived

Feb 24th, 2019–Feb 25th, 2019

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

A persistent weak layer that has been most pronounced at lower elevations has created a low likelihood, high consequence scenario that requires discipline and careful terrain selection to manage effectively.

Confidence

High - The weather pattern is stable

Weather Forecast

SUNDAY NIGHT: Clear / Light, northeasterly winds / Alpine low -19.MONDAY: Sunny / Light, northeasterly winds / Alpine high -17.TUESDAY: Sunny / Light, northeasterly winds / Alpine high -15.WEDNESDAY: Sunny / Light, northeasterly winds / Alpine high -8.

Avalanche Summary

On Saturday there was a report of a skier triggered size 2 persistent slab avalanche 30-50 cm deep on a northeast aspect in the Dogtooth range. This person was seriously injured after being swept over a 30 m cliff and carried by the avalanche for a total of 650 m. On Thursday and Friday a few reports of natural and skier triggered wind slab avalanches to size 1 in the alpine. On Thursday, there was a report of a human triggered persistent slab avalanche, size 1.5. On Wednesday there was a report of a human triggered size 2 persistent slab avalanche in the Gorman Lake area. There is a great MIN report detailing this avalanche here.Most days this week there have been reports of persistent slab avalanches. These avalanches are becoming fewer and further between, but they are still being triggered by people and carry a high consequence.

Snowpack Summary

Depending on location, the snow surface either consists of 15-40 cm of low density snow, wind slabs, or sun crusts on steep south facing terrain. Wind slabs may be found on all aspects due to recent variable wind direction. Lower down there are two layers of of surface hoar (feathery crystals) that were buried at the end of January and mid-January. These layers are around 30 to 80 cm deep and are most prominent at treeline and below. The surface hoar may sit on a crust on south facing slopes.The base of the snowpack is composed of weak and sugary faceted grains that sit on a crust. This weak layer has produced large and destructive avalanches that are sporadic in nature and very difficult to predict. See the Forecaster Blog here for more information on this problem.

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.

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.

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.