Avalanche Forecast

Issued: Feb 21st, 2019 4:38PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Persistent Slabs.

kdevine, Avalanche Canada

Purcells.

Another day of yellow! Though avalanche occurrences are becoming less frequent, the possibility of triggering persistent slabs still exists, and the consequences are high. A cautious approach to backcountry travel is recommended.

Summary

Confidence

High -

Weather Forecast

THURSDAY NIGHT - Cloudy with clear periods and isolated flurries / southwest winds 15-30 km/h / alpine low temperature near -14FRIDAY - Cloudy with scattered flurries, 2-4 cm / southwest winds 20-30 km/h / alpine high temperature near -12SATURDAY - Mainly cloudy with isolated flurries, 2-4 cm / southeast winds 10-15 km/h / alpine high temperature near -12SUNDAY - A mix of sun and cloud / northeast winds, 10-15 km/h / alpine high temperature near -15

Avalanche Summary

On Wednesday, there were a few reports of human triggered avalanches, size 1-1.5 on both north and southerly aspectsOver 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

Recent snow and wind have formed slabs at higher elevations. The most snow accumulated in the south of the region. The wind has blown from variable directions, so all aspects are suspect.

If triggered, wind slabs may step down to deeper layers and result in even larger avalanches.Use caution in freshly wind-loaded features, especially near ridge crests and in steep terrain.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Persistent Slabs

Two weak layers of surface hoar are buried between 30 and 70 cm in the snowpack. The layers are most prominent around treeline and below treeline elevations.

Use conservative route selection, choose moderate angled terrain with low consequence.Avoid steep, open and/or sparsely treed slopes at and below treeline.Be aware of the potential for wide propagation.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1.5 - 2.5

Valid until: Feb 22nd, 2019 2:00PM