Avalanche Forecast

Issued: Feb 28th, 2019 4:51PM

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.

jsmith, Avalanche Canada

Purcells.

Recently formed wind slabs are sitting on top of buried wind slabs that are sitting on facets (weak, sugary snow). The fresh wind slabs may still be touchy and the buried wind slabs are remaining reactive to human triggers longer than is typical.

Summary

Confidence

High - The weather pattern is stable

Weather Forecast

THURSDAY NIGHT: Cloudy with isolated flurries; 0-3 cm. / Light, northwesterly winds / Alpine low -16.FRIDAY: Cloudy with isolated flurries; 1-5 cm. / Light, southwesterly winds / Alpine high -15.SATURDAY: Sunny / Light, northwesterly winds / Alpine high -22.SUNDAY: Sunny / Light, northeasterly winds / Alpine high -18.

Avalanche Summary

On Wednesday, numerous natural wind slab avalanches up to size 2 and several human triggered size 1-1.5s were reported on primarily southern aspects at treeline and above. Here is a link to a video of an avalanche that apparently occurred a few days ago in the Purcells. Great footage that illustrates the current wind slab problem in the region.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. Reports of persistent slab avalanches are becoming less frequent which is creating a low likelihood, high consequence scenario in the snowpack that requires discipline and careful terrain selection to manage effectively.

Snowpack Summary

Strong northeasterly ridge-top winds have created fresh wind slabs on lee features at treeline and above. Additionally, there are buried wind slabs on a variety of aspects that are sitting on facets (sugary snow) which may cause them to remain reactive to human triggers longer than what is typical for a wind slab problem.Lower down there are two layers of surface hoar (weak, 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 in specific locations which is a dangerous combination.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

Wind Slabs

Buried, hard wind slabs that often become unreactive after a few days continue to surprise riders. Thin to thick rocky areas are likely places to trigger these.

Avoid freshly wind-loaded features, especially near ridge crests and in steep terrain.Avoid slopes that sound hollow or drum-like.Steep and rocky terrain are likely places to trigger buried wind slabs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

Two weak layers of surface hoar are buried between 30 and 70 cm down in the snowpack. These layers are slow to change and have created a low likelihood, high consequence scenario in the snowpack.

Avoid low elevation cut-blocks where this layer is well preserved.Avoid steep, open and/or sparsely treed slopes at and below treeline.Use conservative route selection, choose moderate angled terrain with low consequence.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 2.5

Valid until: Mar 1st, 2019 2:00PM