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Avalanche Forecast

Archived

Feb 20th, 2019–Feb 21st, 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 activity has slowed down, it is worth keeping in mind that the potential still exists for humans to trigger persistent slab avalanches. While these avalanches are becoming less likely, the consequences remain high.

Confidence

High -

Weather Forecast

WEDNESDAY NIGHT - Cloudy with clear periods and isolated flurries / northeast winds 10-20 km/h / alpine low temperature near -17THURSDAY - A mix of sun and cloud with isolated flurries / southeast winds 10-20 km/h / alpine high temperature near -13FRIDAY - Mainly cloudy with flurries, 5 cm / southwest winds 15-30 km/h / alpine high temperature near -13SATURDAY - A mix of sun and cloud with isolated flurries / northwest winds, 10-15 km/h / alpine high temperature near -13

Avalanche Summary

On Tuesday, one size 2 natural avalanche was reported on a southeast aspect.On Monday, there were reports of several natural and human triggered avalanches to size 2.5 on both north and south aspects at alpine and treeline elevations.On Sunday, there were reports of both natural and human triggered avalanches to size 2. Most of these were either storm slabs or wind slabs, but a few of them were persistent slabs that failed on a persistent weak layer that was buried in mid January. 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.