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Avalanche Forecast

Archived

Feb 22nd, 2019–Feb 23rd, 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.

Though avalanche occurrences are becoming less frequent, the possibility of triggering persistent slabs and wind slabs still exists. A cautious approach to backcountry travel is recommended.

Confidence

High -

Weather Forecast

FRIDAY NIGHT - Mainly cloudy with isolated flurries, 2-3 cm  / southwest winds 10-15 km/h / alpine low temperature near -15SATURDAY - Cloudy with scattered flurries, 2-3 cm / southeast winds 10-15 km/h / alpine high temperature near -12SUNDAY - A mix of sun and cloud / east winds 10-15 km/h / alpine high temperature near -15MONDAY - A mix of sun and cloud / northeast winds, 10-15 km/h / alpine high temperature near -14

Avalanche Summary

On Thursday, there was a report of a human triggered persistent slab avalanche, size 1.5.  There were also a few reports of natural and human triggered wind slab avalanches to size 1.On Wednesday, there were a few reports of human triggered avalanches, size 1-1.5 on both north and southerly aspects. Of note 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.Over the past week there have been daily reports of natural and human triggered wind slab avalanches in the Purcells. Most days there have also been reports of persistent slab avalanches failing on a weak layer buried mid January. This layer is discussed in more detail in the Snowpack Summary below. These avalanches are becoming fewer and further between, but they are still being triggered by humans and they and carry high consequences as they tend to be large avalanches.There is a great MIN report of a human triggered persistent slab avalanche on Sunday in the Hellroaring Creek area here.

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. This new snow sits on old wind slabs and/or facets (sugary snow).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 70 cm deep and are most prominent at treeline and below treeline elevations. 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. It is most likely to be triggered from areas where the snowpack is shallow and weak. Rocky alpine bowls, ridge crests, and rocky outcroppings are some examples of terrain features to be wary of. See the Forecaster Blog here for more information on this problem.

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.

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.